For utilizing silicon photomultipliers (SiPMs) in space experiments, we conducted irradiation experiments of SiPMs with protons and iron ions. In particular, the relationship between the leakage current and the radiation dose was investigated under various irradiation conditions to determine techniques of mitigating damage caused by radiation. The leakage current was approximately proportional to the dose; however, the proportionality constant did not significantly vary with the irradiation conditions. The effects of annealing after irradiation tests indicated no substantial decrease in the leakage current. Conversely, while the leakage current increased with the dose, the SiPMs continued to operate even after 5 krad of irradiation, which corresponds to several years of radiation exposure in space at altitudes of ~400 km to 500 km. The results obtained from these irradiation tests are used to characterize a hard X-ray (> 60 keV) detector for space-based applications. This detector couples an inorganic scintillator with a high light yield, such as CsI(Tl), to a SiPM.