PURPOSE: To detect a motor lock quickly by detecting a position through generating a pulse, whenever the movable part of DC motor moves at a specified distance, and by inputting said pulse to two charge-and-discharge circuits with a specified time constant.

CONSTITUTION: One of both ends of a moving-coil type linear DC motor 21 is connected with the positive electrode of a power supply 23 via relay bridge circuit 22 and the other thereof is earthed further via a CT 24, FET 25. Said CT 24 generates a pulse, whenever the movable part of DC motor 21 moves at a specified distance. Said pulse of the CT 24 is waveform-shaped 26 and inputted to charge-and-discharge circuits 27, 28 with a specified time constant. In said circuit 27, a charged capacitor C1 discharges for every pulse from the CT 24. In the circuit 28, a charged capacitor C2 normally discharges in the same way as the capacitor C1, but when the pulse does not come from the CT 24, the voltage of said capacitor C2 rises and an exciting stop circuit 29 outputs power to cut OFF FET 25.