There is provided a musical composition data editing apparatus which enables the generation of musical composition data using which a user can cause his/her own musical image to be reflected to a high degree in a musical composition performance through simple operations, whereby a wide variety of users from beginners having no knowledge of musical composition data such as MIDI data to experts who want to arrange musical compositions in their own manner can be satisfied. A standard musical composition data generating section 230 of a contents server CS refers to musical composition reproduction control codes for controlling reproduction of a musical composition contained in source musical composition data, to determine whether or not a musical image in the musical composition reproduction changes. When it is determined that the musical image in the musical composition reproduction changes, rewriting of the musical composition reproduction control codes is carried out such that a musical composition reproduction state becomes constant.