Background: HIV infections among men who have sex with men (MSM) are increasing in Japan. Method: An anonymous self-administered questionnaire among clients of a gay bar in Osaka was used to analyse the relationship between age and unprotected anal sex (UAI). Results: The highest rate of UAI was reported among those aged ≥45 (73.3%), followed by the ≤24 (60.7%), 25-34 (56.3%) and 35-44 (54.0%) age groups (P≤0.01). In multivariate analysis, UAI was related to sex with six or more sexual partners among those aged ≤24 (adjusted odds ratio (AOR), 4.88; confidence interval (CI), 1.21-19.74), bisexual identity (AOR, 2.47; CI, 1.06-5.76) and drug use (AOR, 0.49; CI, 0.26-0.93 for no drug use) in the 25-34 age group, and no lifetime HIV testing in the 3544 age group (AOR, 2.57; CI, 1.40-4.74). Condom purchasing and condom carrying were protective of UAI in 25-34, 35-44 and ≥45 age groups. Conclusion: Programs promoting condom use are needed for younger and older MSM.