In this paper, we propose a composition-by-humming system, called Hummi-Com, that automatically corrects musically inappropriate notes. Although various systems with a composition-by-humming function have been developed, it is difficult in practice for non-musicians to compose musically appropriate melodies with these systems due to the target user's insufficient skill at controlling pitch. In this paper, we propose note correction rules for avoiding such musically inappropriate outputs. This rule set is designed based on the idea of searching for a reasonable trade-off between removing dissonant nonchord tones and retaining musically acceptable nonchord tones.