A user designates any one of a plurality of types of key ranges via a key range designating switch and designates, for example via a keyboard, a desired key position to be set as a boundary of the designated key range. In response to such user's operations, an actual key range of the designated key range is determined in accordance with the designated boundary key position, and the keyboard is set in a divided usage state, where the keyboard is usable divided in a plurality of key ranges, on the basis of the determined actual key range. For example, once a left (low-pitch-side) key range is designated, the designated boundary key position defines the upper limit of the left key range, while, once a right (high-pitch-side) key range is designated, the designated boundary key position defines the lower limit of the right key range.