A starter motor control device comprising a starter motor for starting an engine, an auxiliary switch which can interrupt the energization to the starter motor, a circuit for detecting the rotation speed of the engine or other mechanism related to the rotation speed of the engine to output a signal for preventing the starter motor from being conversely driven by the started engine, a circuit for restraining the starter motor from being energized again for a predetermined time just after the energize to the starter motor is cutoff, a first detecting means having a first set value for judging the voltage at an auxiliary switch driving terminal of a switching element for driving the auxiliary switch, a second detecting means having a second set value for judging the voltage of the auxiliary switch driving terminal, and a circuit for determining the operation of the switching element based on the judgements given by the first and the second detection means.