Objectives To examine the effectiveness of a community diagnosis workshop for the staff of a community comprehensive support center (CCSC).<br/>Methods The general instructional objective for the participants of the workshop was to experience the process of community diagnosis and obtain skills that are relevant to implementing community diagnosis. Specific behavioral objectives included the ability to explain necessary processes of community diagnosis and to implement community diagnosis in their daily work in the CCSC. The workshop included 6 sessions conducted for 19 staff of the CCSCs in Suginami ward, Tokyo from May to October, 2011. Workshops were developed based on the Community as Partner model. Group discussions were held among the participants during each session to promote the sharing of opinions. The participants completed 4 evaluations as follows: process evaluation (assessment of the participation rate in each session and content of the sessions), output evaluation (assessment of the dropout rate), quantitative outcome evaluation (comparison of perceptions and behaviors regarding performing community diagnosis at 3 time points; before, shortly after, and at 4 months after the workshop), and qualitative outcome evaluation (open-ended feedback about the workshop). The quantitative outcome evaluation had 2 specific behavioral objectives: (1) evaluation of the concrete image of community diagnosis process and (2) the priority for performing community diagnosis in their daily CCSC work. In addition, we assessed self-efficacy for performing community diagnosis in the daily work.<br/>Results Participation rate was high in every session (83–100%). Almost all participants evaluated the contents of the workshop as understandable, interesting, appropriate, and useful. In total, 18 out of the 19 participants completed the workshop (dropout rate was 5%; 1 took a leave of absence). Thirteen attended the workshop perfectly, but 5 were absent once. Images of the 2 processes of community diagnosis (“the assessment of the problems in the community” and “the solution planning to the problems”) were enhanced before and shortly after the workshop. These effects remained at 4 months after the workshop. The self-efficacy for implementing these 2 processes remained unchanged before and shortly after the workshop, but was improved at 4 months after the workshop. Comparing shortly after and 4 months after the workshop, planning solutions to the problems became a higher-priority task in the participants’ daily work. From the open-ended feedback, the workshop could have had some spillover effect on other aspects of the CCSC. Participants indicated that they could understand the process and significance of community diagnosis.<br/>Conclusion As demonstrated by the staff of a CCSC, the effectiveness of the community diagnosis workshop could have an effect on the individual participant's perceptions and behaviors as well as on the activities of the CCSC.