It is empirically said that music which has the similar characteristics to the negative mood in listening is effective in relaxation (ISO PRINCIPLE, Altshuler, 1954). However, previous studies suggested that the types of music influence relaxation; negative toned music elicited negative emotion. This finding couldnot explain the effect on relaxation by Iso Principle. The purpose of this study was to examine (1) what type of music a subject in what kind of mood feels the similarity between music and mood, (2) whether music which a subject feels the similarity induces relaxation. Thirty-two subjects rated emotional state before and after listening and music impressions after music under the four experimental conditions; Dark-Powerful music (D-P), Dark-Calm music (D-C), Bright-Powerful music (B-P) and Bright-Calm (B-C) music conditions. The results were as follows. (1) Bright and Calm music enhanced positive emotion while Dark music enhanced negative emotion. (2) Subjects who had negative emotion before listening felt the similarity between Dark music and their moods. (3) Subjects who felt high similarity with D-C music sympathized with the music and vented after listening. (4) Subjects who felt high similarity with D-P music were hard to decrease positive moods. These results indicated that the music which has the similar characteristics to the negative mood might be useful to make person who has negtive mood relax.