The purpose of this study was to develop and evaluate a self-efficacy instrument for Japanese obstructive sleep apnea (OSA) patients treated with continuous positive airway pressure (CPAP). Analyzed subjects were 653 Japanese OSA patients (619 males and 34 females) treated with CPAP at a sleep laboratory in a respiratory clinic in a Japanese city. Based on Bandura's social cognitive theory, the CPAP Self-Efficacy Questionnaire for Sleep Apnea in Japanese (CSESA-J) was developed by a focus group of experts, using a group interview of OSA patients for the items of two previous self-efficacy scales for Western sleep apnea patients receiving CPAP treatment. CSESA-J has two subscales, one for self-efficacy and the other for outcome expectancy, and consists of a total of 15 items. Content validity was confirmed by the focus group. Confirmatory factor analysis showed that the factor loadings of self-efficacy and outcome expectancy were 0.47-0.76 and 0.41-0.92, respectively, for the corresponding items. CSESA-J had a significant but weak positive association with the General Self-Efficacy Scale, and a strong positive association with "Self-efficacy scale on health behavior in patients with chronic disease." Cronbach's alpha coefficient was 0.85 for the self-efficacy subscale and 0.89 for the outcome expectancy subscale. The intraclass correlation coefficient using data from the first and second measurements with CSESA-J for a subset of 130 subjects was 0.93 for the self-efficacy and outcome expectancy subscales. These results support CSESA-J as a reliable and valid instrument for measuring the self-efficacy of Japanese OSA patients treated with CPAP. Further studies are warranted to confirm validity for female OSA patients and generalizability.