The prevalence of syphilis is very high in human immunodeficiency virus (HIV)-positive men who have sex with men (MSM), and effective interventions are needed to educate HIV-positive individuals about behavioral and biological risk factors. Therefore, we developed a standard case management process and conducted a randomized controlled study to investigate the impact on risky sexual behaviors and syphilis in HIV-positive MSM. Men who have sex with men (n = 220) were enrolled and randomized to the case management intervention group and the control group between May 2016 and January 2017. The control group received routine HIV-related care. In addition to routine HIV-related care, those in the intervention group regularly received extended services from a well-trained case manager. Epidemiological information was collected during the baseline face-to-face interviews by a trained investigator. Serological tests for syphilis and assessments of risky sexual behaviors were performed at baseline and 6 and 12 months after the initiation of treatment. The syphilis incidence rates in the intervention and control groups were 11.3 per 100 person-years and 20.6 per 100 person-years, respectively. The multivariable-adjusted hazard ratio (95% confidence inter) for syphilis in case management group was 0.34 (0.14-0.87). The percentages of participants who resumed risky sexual behaviors in both groups were significantly reduced (P < 0.05) but did not significantly differ between the 2 groups. A case management intervention reduced the incidence of syphilis in HIV-positive MSM. We should further increase the content of case management on the basis of providing routine HIV-related care to those people.