An on-vehicle sound instrument with a face plate which does not require any switch mounted thereon to improve portability is not started unless the face plate is mounted on an instrument body, and even if any switch in the instrument body is manipulated. The face plate (30) detachably mounted on the instrument body (10) does not have any switch mounted thereon and includes operation keys (320). The instrument body (10) includes a microcomputer (40) for controlling sound signal outputs and adjustments and an input unit (44) formed of switches for inputting various data to the microcomputer (40) through inputs from the operation keys (320). When the face plate (30) is removed from the instrument body (10) and, for example, a power switch is turned on, a start setting device (60) prevents the microcomputer (40) from activating. The start setting device (60) includes two switches (62, 64) which are simultaneously turned on when the face plate (30) is dismounted from the instrument body (10). When both switches (62, 64) are in their ON state, the chip enable terminal of the microcomputer (40) becomes low. As a result, the microcomputer (40) is not started to activate the instrument body (10).