Antiresorptive agent-related osteonecrosis of the jaw (ARONJ) is a severe adverse event associated with use of bone resorption inhibitors (BRIs), such as zoledronic acid and denosumab. Based on the results of phase 3 clinical trials for BRIs, the frequency of ARONJ is reported to be 1 to 2%, but the actual frequency could be higher. We investigated 173 patients with prostate cancer with bone metastases who were treated either with zoledronic acid or denosumab at our hospital between July 2006 and June 2020. ARONJ occurred in 13 patients (8%); i.e., ten out of 159 patients (6%) who were treated with zoledronic acid, and three out of 14 patients (21%) who were treated with denosumab. Multivariate analysis showed that longer duration of BRI exposure and dental treatment before the initiation of BRI are associated with risk of ARONJ. ARONJ is associated with decreased mortality but the association is not significant. Generally, the occurrence of ARONJ may be underestimated; therefore, further studies are warranted to determine the actual frequency of ARONJ.