An actuator comprises: a tubular yoke (31); an armature (130) that is provided on the inner side of the yoke (31) in the radial direction, and that is provided so as to be rotatable relative to the yoke (31); a screw shaft (60) that is rotated and driven by receiving rotary force of a shaft (33) of the armature (130); a speed reduction gear part (50) that is provided between the shaft (33) and thescrew shaft (60), and that reduces the rotation speed of the shaft (33); and a nut member that is cooperatively coupled to the screw shaft (60), and that moves along the axial direction of the screwshaft (60) in association with the rotation of the screw shaft (60). An internal gear (51) of the speed reduction gear part (50) is provided on the inner circumferential surface (31f) of the yoke (31), and a bearing (38B) for rotatably supporting the shaft (33) is provided to the internal gear (51).