The purpose of this study was to verify the mediating effect of self-efficacy in the relationship between attachment-anxiety, attachment-avoidance, and development of counselors, which are unstable adult-attachments of counselors. To this end, attachment-anxiety, attachment-avoidance, self-efficacy, and level of development were measured with a self-report questionnaire for 193 counselors participating in the H Counseling Society. For this purpose, 193 counselors from the H Counseling Association were measured with self-reporting questionnaires for attachment-anxiety, attachment-avoidance, self-efficacy, Development Levels. Data analysis was performed using SPSS 21.0 and correlation and mediated effects were analyzed. The results of the study are as follows. First, the counselor s attachment-anxiety and anxiety-avoidance were negatively correlated with self-efficacy and developmental level, and self-efficacy and developmental level were positively correlated. Second, in the relationship between the counselor s attachment anxiety and developmental level, the full mediating effect of self-efficacy was verified. Third, in the relationship between the counselor s attachment- avoidance and developmental level, the full mediating effect of self-efficacy was verified. Fourth, in the relationship between the counselor s unstable adult-attachment and the level of development, the complete mediating effect of self-efficacy was verified. Therefore, in order to offset the counselor s insecure adult attachment and promote the counselor s development, it should be applied to research, education, and training with a focus on self-efficacy intervention. Finally, we discussed the results of this study, suggested limitations, and suggested further research.