PURPOSE: To shorten overall length, to decrease the number of component and to simplify assembly work by fixing a resistor substrate onto the base of the inside of a gear case while also fastening a bearing on the opposed face side with the resistor substrate of the gear case and directly engaging a brush mounting plate with a friction disc.

CONSTITUTION: A shaft 21 is held rotatably to a bearing 22, a brush 24 is caulked, and a brush mounting plate 23, into which a friction disc 25, a frictional gear 26 and a coil spring 27 are inserted, is installed. A resistor substrate 30 is fixed onto the base of a gear case 31, a worm gear 32 is held rotatably to the resistor substrate 30, and the worm gear 32 is engaged with the frictional gear 26. A helical gear 35 is connected to a motor 36 fastened to the gear case 31, and engaged with the worm gear 32. The bearing 22 is mounted on the opposed face side with the resistor substrate 30 of the gear case 31. The helical gear 35 is turned by the revolution of the motor 36 for drive and the worm gear 32 is interlocked and decelerated, the rotation of the frictional gear 26 and the friction disc 25 is transmitted over the brush mounting plate 23, and the brush 24 is rotated and slid on the resistor substrate 30.