PURPOSE: To make the constitution simple, adjust a caster angle by moving an upper suspension arm longitudinal, and thereby enhance operating performance.

CONSTITUTION: A housing 9 carried by a vehicle frame 20 is provided with a cylindrical section 9a in the longitudinal direction, and a sleeve 24 is supported by the cylindrical section 9a in such a way as to be rotatable but uncapable in moving in the axial direction. A worm shaft 26 is threadedly engaged with a gear 24a formed over the outer circumferential surface of the sleeve 24. The base end flanges 8a of an upper suspension arm 8 are combined with both end sections of a screw shaft 21 threadedly engaged with the sleeve 24. The worm shaft 26 is rotated by the output of an electronic control means 32 based on a signal from a vehicle sensor 31.