Background: The oligometastatic cancer has been suggested as an intermediate state between localized disease and wide range metastases. Clinical significance of local treatment in oligometastatic prostate cancer (PCa) has been a recent topic. However, due to racial differences, there are no standard definitions of oligometastasis, especially in Asians. Here we studied the risk factor among Japanese oligometastatic PCa patients. Methods: We retrospectively analyzed the medical record of 207 patients, including locally advanced (T3 or T4N0M0), lymph-node-positive (N1), and ≤10 bone metastases cancer. All patients received androgen deprivation therapy only. The number of bone metastasis and clinical factors were evaluated in association with OS. Results: Median age, PSA at baseline and OS were 73 years, 48.8ng/ml and 121 months respectively. The cutoff value for the number of bone metastases that have the greatest impact on OS was ≥2 (HR 3.05, p = 0.0001), followed by ≥7 (HR 3.03, p = 0.0005). In multivariate analysis, lymph node metastasis (HR 2.54, p = 0.003) and ≥2 bone metastases (HR 2.67, p = 0.009) were independent predictors of OS. In risk classification based on independent predictors, OS was significantly classified in High (3-4 factors), Intermediate (1-2 factors), and Low (no factor) risk groups (p <0.0001). Furthermore, even among same risk factors group, inclusion of lymph node metastasis significantly increased the HR by 2.34 (p = 0.039). Conclusions: Not only the number of bone metastasis but also lymph node metastasis predict OS of oligometastatic PCa patients.