A music data processing method includes estimating a performance position within a musical piece, and updating a tempo designated by music data representing a performance content of the musical piece such that a tempo trajectory corresponds to a transition in a degree of dispersion of a performance tempo, which is generated as a result of estimating the performance position with respect to a plurality of performances of the musical piece, and the transition in the degree of dispersion of a reference tempo. The performance tempo is preferentially reflected in a portion of the musical piece in which the degree of dispersion of the performance tempo falls below the degree of dispersion of the reference tempo, and the reference tempo is preferentially reflected in a portion of the musical piece in which the degree of dispersion of the performance tempo exceeds the degree of dispersion of the reference tempo.