A starter protective device is provided which is capable of accurately determining the commencement of overrunning (i.e., timing of the commencement of starting of an engine) even in the presence of variations in the cycle of fluctuations in a battery voltage, noise and so on, thereby to prevent a starter from being overrun after starting of an engine in a reliable manner. The starter protective device includes an overrunning determination section 15 for determining when the starter motor 4 commences overrunning; and a starter motor cut-off section for interrupting the main contactor 3 when it is determined that the starter motor 4 commences overrunning. The overrunning determination section 15 detects a change over time of the battery voltage BV after the starter switch 2 has been turned on, and determines that the starter motor 4 commences overrunning when the battery voltage VB remains unchanged without any increase or decrease over a predetermined time T.