The purpose of the study was to find factors that had influence on infidelity. This study also attempted to discover factors that influenced positive marital relationship. The total 20 sessions of family counseling continued from January, 2002 to December, 2002. The researcher repeated consistent comparison to find out redundant concepts in this case. The therapist employed as therapeutic models the MRI\'s interactional model and Murray Bowen\'s family systems model in order to treat the family. Based on these two models, the researcher focused on two major common concepts such as undifferentiation of self and communication problem to analyze factors that affected infidelity. As a result of intervention the client experienced the differentiation of self and changed pattern of communication. With these two factors being changed negative marital relationship turned into positive one. This case study showed the utility of both MRI model and Murray Bowen\'s family systems model as appropriate models for Korean couples\' infidelity case.