Previous researches suggest that STEAM education contains both STEM-based and art-based ones. The purpose of this paper is to describe a design of collaborative creative activity in music and an evaluation method as a means of considering art-based STEAM education which improves students’ self-esteem. We designed a three-hours musical workshop for 20 children who were five graders in an elementary school to facilitate the creation of harmonies to the given melodies. The children used ICT to externalize their ideas in collaborative learning, and they were evaluated pre-, intermediate- and post-test. Each test contains a questionnaire, an exam of creating harmonies, and questions of self-esteem. The results of the questionnaire indicated that all the children enjoyed the workshop and some of them became externalization than appreciation. Furthermore, the children who made not qualified harmonies improved their self-esteem in post-test than pre-test, though the self-esteem of the children who made moderate quality of harmonies decreased. Detailed protocol analysis suggested that the pair of children who improved their self-esteem allocated their time to consider each other’s ideas and not hurried to complete making their harmony. The importance of evaluating the children’s internal change in this workshop as an art-based STEAM education was discussed.