PURPOSE: To enable a valve to be back seated via limit control, prevent the occurrence of a load due to excessive tension and make a motor compact for a valve drive device.

CONSTITUTION: A tapered valve cover side backseat section 8a is formed on the lower surface of the penetration hole of a valve cover 10 for inserting a valve spindle 7, and a spindle side backseat section 9 is formed on the lower section of the spindle 7. Also, a backseat device 20 is provided between the backseat sections 8a and 9. In this case, the device 20 is made of a pair of upper and lower seal rings 16 and 17, and a bellows 13 and a disc spring laid between the seal rings 16 and 17. In addition, a vent hole 19 is fitted with a nonreturn valve 18.