In this retrospective study, we aimed to clarify the risk of developing a second primary cancer and to determine the periods of high risk of second primary cancers. Subjects were all patients who had been diagnosed with a first primary cancer and registered with the Nagasaki Prefecture Cancer Registry between 1985 and 2007. We calculated the standardized incidence ratio (SIR) of second primary cancer according to site and years after diagnosis of the first primary cancer. A second primary cancer developed in 14 167 of 174 477 subjects (8.1%) during a median follow‐up of 1.8 years. The SIR of all cancer was 1.10 (95% confidence interval, 1.08–1.11). Some specific relationships were observed between sites with risk factors in common, such as smoking, drinking, and hormone status. The SIRs were relatively high after approximately 10 years for all sites, and trends differ among cancer sites. We showed that cancer patients are at higher risk of a second primary cancer than the general population. In respect of the risk of a second primary cancer, physicians should be alert for cancers that have risk factors in common with the first primary cancer. We examined the risk of developing a second primary cancer and the optimal duration of follow‐up for cancer patients in regard to multiple primary cancers. The results showed that medical scrutiny for second primary cancers that have risk factors in common with the first primary cancer is important, and follow‐up for 10 years for some sites is needed.