To make it possible to meticulously control the mode changeover of sound source systems by outputting mode information for controlling the sound source system by the state of a playing mode changeover switch.

Musical tones are generated (1), (2), (3) from the external sound source system A or B or A and B over the entire area of a keyboard when the SW on a turned left side is turned on and further, the SW on the right side of the same system as this system is turned on. When the SWs on the turned on left side and right side are turned on as they are, the musical tones are respectively generated from the external sound source system A or B by the operation of the keys on the left side and the right side from the split points of the keyboard (4). The sound source systems A and B are changed over in this state and the musical tones are generated (5). The musical tones are generated from the external sound source system A by the operation of the left side keys from the split point of the keyboard and the musical tones are generated from the external sound source systems A and B by the operation of the keys on the right side (6).