Article 37 (3) of the Constitution of Japan, modeled on the Sixth Amendment of the Constitution of the United States, is widely believed to guarantee the defendant the right to the effective assistance of counsel. Despite such belief, Japanese courts have never applied the provision to actual cases in which individual defendants sought remedies for deprivation of their right to effective assistance. This article shows how the effective assistance guarantee of the Sixth Amendment developed over a century prior to Strickland v. Washington, a U.S. Supreme Court decision that establishes the standard for evaluating counsel’s ineffective performance, and identifies the Court’s understanding of the vital importance of the defense function that underlies the constitutional guarantee. The article also examines a series of Japanese Supreme Court decisions on the right to counsel and makes it clear that the Japanese Court does not regard defense counsel as a crucial player of the adversarial system, i.e., it does not share the same idea as the U.S. Supreme Court holds about the role of counsel as the foundation of the constitutional guarantee. Thus, establishment of the right to the effective assistance of counsel in Japanese courts requires a change of the judiciary’s basic attitude toward the defense function.