The purpose of this study is to empirically clarify how we should design a professional development program for textbook developers by investigating a case of social studies textbook development training course in Japan implemented in June, 2018. We evaluated the outcomes and issues of the case as a design-based research by applying the three-layer conceptual model (“Skill”, “Theory”, and “Belief”) of the expertise for textbook developers and five design principles for the professional development program focusing on “Rationale Development”. The results show the autonomous and positive change in belief of the trainees as well as the improvement of a certain degree of cognitive understanding in theory, and practical skills of textbook design. By disclosing and systematizing the tacit knowledge of such professional development program, this study implies (i) the measures of evaluating and improving the professional development program and its design principles for practitioners in international cooperation, (i) the assurance of mutuality between trainers and trainees, and opportunity of meta-cognition on learning process and outcomes for trainees, and (iii) the potential application of Japan’s experience in textbook development for educational researchers in the field of curriculum and instruction.