In human-robot interaction, how to express robot's emotion to human is an important issue. In this paper, we have proposed the music as a medium expressing robot's emotion. Since emotions and the feelings from music are somewhat subjective, we have composed the music considering musical factors to reduce ambiguity. As an earlier work, we composed the music for happy, sad, fear, and dislike. We analyzed their musical factors and explained the motives of composition. To validate the effect of the composed music for happy, sad, fear, and dislike, we have conducted experiments and compared the results with that on robot's facial expression. The results showed that the composed music is very effective in conveying emotions. In addition, the results showed better emotion expression when music and facial expression were simultaneously presented.