A flow of hydraulic oil induced by sliding movement of a piston in a cylinder is controlled by a damping force generating mechanism to generate a damping force. A flow of hydraulic oil from a casing of the damping force generating mechanism toward a reservoir is regulated by a passage groove formed on the bottom of the casing so as to be directed downward in the reservoir. Thus, the hydraulic oil is efficiently supplied to a base valve from the damping force generating mechanism even when the hydraulic oil is sucked from the reservoir into a cylinder lower chamber through a passage in the base valve during the extension stroke of a piston rod. Therefore, it is possible to suppress a sharp reduction in pressure in the lower part of the reservoir and hence possible to suppress the occurrence of aeration and to obtain stable damping force characteristics.