The purpose of this study is to examine the moderating effects of supervisory relationship between play therapist's developmental level and countertransference management ability. The subjects of this study were 165 play therapists with supervision experience. The results of this study can be summarized as follows. First, as a result of a correlation analysis on the relationship between play therapist's developmental level, supervisory relationship (safe base, reflective education, structure) and countertransference management ability all showed a significant and positive correlation to each other. Second, as a result of hierarchical multiple regression analysis conducted to examine the moderating effects of supervisory relationship (safe base, reflective education, structure) between play therapist's developmental level and countertransference management ability, safe base, reflective education, structure showed significant moderating effects on the relationship between play therapist's developmental level and countertransference management ability. In this study, it is meaningful to expand the study on the play therapist's ability to manage countertransference by examining the moderating effect of the supervisory relationship in the relationship between play therapist's developmental level and countertransference management ability.