diff --git a/README.md b/README.md
index af1fc2e..9100c9d 100644
--- a/README.md
+++ b/README.md
@@ -46,7 +46,7 @@ C++17
    + ✅ Special operator `IF`
    + macro `DO`
    
- - Tail Call Optimization (TCO). 実装したがテスト未了
+ - ✅ Tail Call Optimization (TCO). トランポリン trampoline で相互再帰もOK.
  
  - Files
  
diff --git a/environment.cpp b/environment.cpp
index 80f2ef1..853064a 100644
--- a/environment.cpp
+++ b/environment.cpp
@@ -101,9 +101,9 @@ FuncPtr Environment::find_function(const UnicodeString& symbol)
 void Environment::set_value(const UnicodeString& symbol, const value_t& value,
                             bool constant)
 {
-    std::cout << __func__ << " at " << __LINE__ << ": ";
-    std::cout << symbol << " = " ;
-    PRINT(value, std::cout); std::cout << "\n"; // DEBUG
+    //std::cout << __func__ << " at " << __LINE__ << ": ";
+    //std::cout << symbol << " = " ;
+    //PRINT(value, std::cout); std::cout << "\n"; // DEBUG
 
     // insert() は更新しない!
     m_values[symbol] = BoundValue { .val = value, .constant = constant};
diff --git a/my_debug.h b/my_debug.h
index 2e3ac45..25503a8 100644
--- a/my_debug.h
+++ b/my_debug.h
@@ -11,7 +11,7 @@
 
 // 不要のときはコメントアウト
 //#define DEBUG_OBJECT_LIFETIMES         1
-#define DEBUG_ENV_LIFETIMES            1
+//#define DEBUG_ENV_LIFETIMES            1
 #define DEBUG_TRACE_FILE    stderr
 
 
diff --git a/test/Makefile b/test/Makefile
index fcf9793..da3bcd4 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -2,7 +2,7 @@
 .PHONY: all clean
 
 TARGETS = ref_counted_test  reader_test  edit_line_test environment_test \
-    macro_test evaluation_test block_test
+    macro_test evaluation_test block_test tco_test
 
 all: $(TARGETS)
 
@@ -49,6 +49,9 @@ macro_test: macro_test.o ../macros.o ../environment.o ../object_print.o ../reade
 evaluation_test: evaluation_test.o ../evaluation.o ../environment.o ../object_print.o ../reader.o ../macros.o ../builtin-functions.o
 	$(CXX) $^ $(LDFLAGS) $(LDLIBS) -lstdc++ -licuuc -licuio -ledit -o $@
 
+tco_test: tco_test.o ../evaluation.o ../environment.o ../object_print.o ../reader.o ../macros.o ../builtin-functions.o
+	$(CXX) $^ $(LDFLAGS) $(LDLIBS) -lstdc++ -licuuc -licuio -ledit -o $@
+
 block_test: block_test.o ../evaluation.o ../environment.o ../object_print.o ../reader.o ../macros.o ../builtin-functions.o
 	$(CXX) $^ $(LDFLAGS) $(LDLIBS) -lstdc++ -licuuc -licuio -ledit -o $@
 
diff --git a/test/tco.cpp b/test/tco.cpp
new file mode 100644
index 0000000..795dfd6
--- /dev/null
+++ b/test/tco.cpp
@@ -0,0 +1,27 @@
+
+#include <iostream>
+using namespace std;
+
+// -S オプションでアセンブラ出力
+
+bool isodd(int n) ;
+
+// 偶数のとき true
+bool iseven(int n) {
+    return n > 0 ? isodd(n - 1) : true;
+}
+
+// 奇数のとき true
+bool isodd(int n) {
+    return n > 0 ? iseven(n - 1) : false;
+}
+
+int main()
+{
+    // オプションなし = segfault, -O3  => 結果 1 OK
+    cout << "iseven(1'000'000) = " << iseven(1'000'000) << "\n";
+
+    // ちょっとだけ間があく = 計算したうえで, 結果 1 OK
+    cout << "isodd(2'000'000'001) = " << isodd(2'000'000'001) << "\n";
+    return 0;
+}
diff --git a/test/tco_test.cpp b/test/tco_test.cpp
new file mode 100644
index 0000000..55761ea
--- /dev/null
+++ b/test/tco_test.cpp
@@ -0,0 +1,70 @@
+
+#include "../environment.h"
+#include <sstream>
+#include <iostream>
+#include <fstream>
+
+namespace my {
+extern value_t EVAL1(value_t ast, EnvPtr env);
+extern bool value_isTrue(const value_t& value) ;
+
+extern void setup_functions();
+
+my::value_t do_1minus(my::EnvPtr args) {
+    my::value_t x = args->find_value("X");
+    double v = std::get<double>(x);
+
+    return v - 1;
+}
+
+// @return T or NIL
+my::value_t do_gt(my::EnvPtr args) {
+    double x = std::get<double>(args->find_value("X"));
+    double y = std::get<double>(args->find_value("Y"));
+    return x > y ? trueValue : nilValue;
+}
+
+} // namespace my
+
+
+int main()
+{
+    my::EnvPtr env = std::make_shared<my::Environment>();
+
+    my::setup_functions();
+    my::define_function("1-", "(x)", my::do_1minus);
+    my::define_function(">", "(x y)", my::do_gt);
+    my::globalEnv->set_value("T", my::trueValue, true);
+    my::globalEnv->set_value("NIL", my::nilValue, true);
+
+    // 偶数 = true
+    icu::UnicodeString ast = "(defun iseven (n) (if (> n 0) (isodd (1- n)) t))";
+    my::value_t astv = my::read_from_string(ast);
+    my::EVAL1(astv, env);
+
+    // 奇数 = true
+    ast = "(defun isodd (n) (if (> n 0) (iseven (1- n)) nil))";
+    astv = my::read_from_string(ast);
+    my::EVAL1(astv, env);
+
+    ast = "(print (iseven 10))";
+    astv = my::read_from_string(ast);
+    my::EVAL1(astv, env); //=> T
+
+    ast = "(print (isodd 10))";
+    astv = my::read_from_string(ast);
+    my::EVAL1(astv, env);  //=> NIL
+
+    ast = "(print (iseven 9))";
+    astv = my::read_from_string(ast);
+    my::EVAL1(astv, env);  //=> NIL
+
+    ast = "(print (isodd 9))";
+    astv = my::read_from_string(ast);
+    my::EVAL1(astv, env); //=> T
+
+    astv = my::read_from_string("(print (iseven 1000000))");
+    my::EVAL1(astv, env);  //=> T
+
+    return 0;
+}