Moral disengagement is a cognitive distortion that makes it easy to behave immorally and explains how people who internalize moral standards violate them while avoiding conflict and guilt. This study aimed to develop a Japanese version of the Moral Disengagement Scale (J-MDS) to measure individuals' tendency toward moral disengagement and examine its reliability and validity. In Study 1, exploratory and confirmatory factor analyses confirmed the validity of the single-factor structure of the 8-item moral disengagement scale. Additionally, the J-MDS negatively correlated with moral identity internalization and proneness to guilts. Study 2 showed that the J-MDS significantly predicted immoral behavior. Study 3 confirmed the incremental validity of the J-MDS. Study 4 confirmed the test-retest reliability of the J-MDS. Study 5 showed that the J-MDS negatively correlated with internal attribution and positively correlated with external attribution. These results indicate that the J-MDS has a certain degree of reliability and validity.