PURPOSE: To increase the number of revolutions at high speed of a rotary drive unit employed in a soil improving machine or the like to rotate an auger screw or the like by providing two two-speed motors each having a high-to-low speed switch, and also providing a series-parallel switch, etc., by which both of the two-speed motors are connected in series or in parallel.

CONSTITUTION: When high-to-low speed switches 5 used in a pair of two-speed motors 2, 2 are switched to the low-speed side (right position) and a series- parallel switch 3 opens a parallel side solenoid valve 8 and closes solenoid valves 9, 10, both of the two-speed motors 2 are rotated at low speed, and a main spindle 11 is rotated at low speed via a transmission mechanism 4. Then when each high-to-low speed switch 5 is switched to the low-speed side and the series-parallel switch 3 opens a series-side solenoid valve 8 and closes the solenoid valves 9, 10, both of the two-speed motors 2 are rotated at medium speed. Also when both of the high-to-low speed switches 5 are switched to the high speed side and the series-parallel switch 3 is switched to the series side, both of the two-speed motors 2 are rotated at high speed and the main spindle 11 is rotated at high speed via the transmission mechanism 4.