The purpose of this study was to clarify how Japanese nurse managers (i.e., “shunin”) or higher-ranked positions self-rate their nursing management competencies and to identify factors associated with their ability to develop staff. Data were collected using a questionnaire based on the 41-item Management Index for Nurses. This index assesses the competencies related to six components of nursing management: planning, motivating staff, developing staff, communication, organization, and ensuring safety. The total possible score is 205 points. The mean percentage score for each component was calculated based on the responses from 118 participants (107 women; mean age = 44.1 ± 7.0 years). Results showed that the mean percentage score for competencies related to ensuring safety was, by far, the highest (71.8%), and the lowest was for competencies related to organization (47.6%). Principal factors found to be associated with participants’ ability to develop staff were “gathering and using information” (a subscale of “educational background and interests”) and “supportiveness of the work environment”. These results suggest that, to improve nurse managers’ competencies related to their ability to develop staff, hospitals need to establish continuing education systems that offer nurse managers convenient educational opportunities in management science, either on-site or at a higher education institution; and develop an in-house support system that enables managers to obtain counseling when practical management concerns cause them stress.