<p>Informed consent for clinical trials is often difficult for patients. In particular, placebo-controlled randomized double-blind trials are poorly understood. While many studies have measured patients' understanding, there is no validated scale used in Japan. The “Quality of Informed Consent Scale” (QuIC) is a validated scale used to assess patients' objective and subjective understanding of cancer clinical trials. Therefore, we have translated the QuIC into Japanese and created a modified version for general use in placebo-controlled, randomized, double-blind trials; we subsequently examined the linguistic validity of the modified version of the QuIC. This study was conducted in accordance with the guidelines of the International Society for Pharmacoeconomics and Outcomes Research.</p><p>First, three translators put forward translations from English to Japanese, which were synthesized into a Japanese version and modified through discussions with our research team. The original author confirmed the back-translated Japanese versions, which were refined via feedback from the original author. Furthermore, cognitive debriefing of the modified version was conducted on patients who had undergone a clinical trial.</p><p>Through the procedures outlined by the scale translation guidelines, consultations with the original authors, and cognitive debriefings with actual patients, a Japanese translation consistent with the original version was developed, and the linguistic validity of the modified version was confirmed. Although the reliability and validity of these scales for Japanese patients in clinical trials are yet to examined, this modified QuIC may be useful for measuring understanding in placebo-controlled, randomized, double-blind trials.</p>