A tone generating portion has a performance play capability for concurrently generating, on the basis of a set of performance information, musical tone signals having various voices each assigned to one of parts. A performance data setting portion chooses, under user's direction, one of performance data sets stored in a performance data set storage portion and makes settings of the various voices applied to musical tone signals for the parts. A developing portion enables generation of musical tone signals for at least one of the parts and disables generation of musical tone signals for the other parts in response to user's direction in a state where respective settings of various voices to be applied to musical tone signals for the parts have been done. A tone range defining portion defines tone range data of the at least one part as representing a tone range including at least pitches of all the musical tone signals specified by the performance information.