Patients with bone metastases are treated with long-term bone resorption inhibitors such as bisphosphonates and denosumab. However, resorption inhibitors have been known to cause fractures, such as atypical femoral fractures (AFFs). In recent years, there have been an increasing number of reports of atypical ulna fractures (AUFs) caused by bone resorption inhibitor usage. Treatment of AUFs is complicated, especially when they occur in patients with bone metastases, because it is difficult to discontinue bone resorption inhibitor treatment without the risk of aggravating metastatic lesions. Prophylactic surgery is recommended in AFFs when fractures are predicted, but there are few reports of prophylactic surgery for AUFs. Here, we report a case of incomplete AUF in a 74-year-old woman which was surgically treated with prophylactic plate fixation. The patient had been using denosumab for 6 years to treat bone metastases due to thyroid cancer. After surgery, no fractures were observed for 2 years without discontinuing denosumab, and her forearm function was adequate. AUFs are rare and difficult to treat, so oncologists who treat bone metastases need to pay special attention to diagnose this incomplete AUF before the fracture worsens. We believe that detection of a possible fracture and prophylactic surgery can improve prognosis.