PURPOSE: To eliminate vibrational noise generation from a return spring and also wear thereof, which is to close a throttle valve.

CONSTITUTION: When a throttle valve is opened, a return spring 14 is contracted in the diameter and comes in contact with a projection 17A on a collar 17. The collar 17 decreases in the max. diameter and elongates in the axial direction. The collar 17 is detained with recesses 12C, 13C formed in the first spring guide 12 and second spring guide 13, and because of a play between them, it never stretches against the sides even when elongated in the axial direction. Thus the return spring 14 is prevented from vibrations.