The Implicit Positive and Negative Affect Test (IPANAT) is an instrument for the indirect assessment of positive and negative affect. A Japanese version of the IPANAT was developed and its reliability and validity were examined. In Study 1, factor analysis identified two independent factors that could be interpreted as implicit positive and negative affect, which corresponded to the original version. The Japanese IPANAT also had sufficient internal consistency and acceptable test–retest reliability. In Study 2, we demonstrated that the Japanese IPANAT was associated with explicit state affect (e.g., PANAS), extraversion, and neuroticism, which indicated its adequate construct validity. In Study 3, we examined the extent to which the Japanese IPANAT was sensitive to changes in affect by assessing a set of IPANAT items after the presentation of positive, negative, or neutral photographs. The results indicated that the Japanese IPANAT was sufficiently sensitive to changes in affect resulting from affective stimuli. Taken together, these studies suggest that the Japanese version of the IPANAT is a useful instrument for the indirect assessment of positive and negative affect.