To prevent damage due to leakage current resulting from the adhesion of water to a circuit board.

A drive circuit 1 for a motor key comprises two terminals T1-T1 and T2-T2 to one of which a positive voltage is applied when locked or unlocked, two sets of micro-switches MS1 and MS2 of which contact points are switched in connection with the movement of a dead bolt driven in and out by a motor M, and two sets of transistors TR1 and TR2 which are on/off controlled through interlocking to the switch-over of the two sets of micro-switches MS1 and MS2 so as to switch the direction of electric conduction from the terminals T1-T1 and T2-T2 to the motor M. A circuit board on which the two sets of transistors TR1 and TR2 are mounted is provided with by-pass wiring patterns 11 and 12 which forcibly eliminate leakage current resulting from the adhesion of water to the circuit board when the transistors TR1 and TR2 are in an off state.