A lash adjuster which comprises: a bottomed cylindrical body; a bottomed cylindrical plunger which is movable upward and downward while being in sliding contact with an inner circumferential surface of the body and which is biased upward by a return spring; a low pressure chamber defined in the plunger for storing an operating fluid; a high pressure chamber defined in a hollow lower interior of the body and separated from the low pressure chamber by a bottom wall of the plunger, the high pressure chamber being filled with the operating fluid; and a check valve which allows the operating fluid to flow from the low pressure chamber into the high pressure chamber when in a valve-opened state and which cuts off flow of the operating fluid from the high pressure chamber into the low pressure chamber when in a valve-closed state, the check valve including a valve orifice extending through the bottom wall of the plunger, a retainer disposed in the high pressure chamber so as to be moved upward and downward with the plunger, and a valve element which is vertically movable in the high pressure chamber between a valve-opened position at which the valve element is placed on the retainer while spaced from the valve orifice and a valve-closed position which is located higher than the valve-opened position and at which the valve element closes the valve orifice, the valve element being held in the valve-opened position by self-weight when the plunger remains still, wherein the retainer includes: a base plate on which the valve element located at the valve-opened position is placed; and a cylindrical portion extending upward from an outer circumferential edge of the base plate so as to surround the valve element; the cylindrical portion having an opening formed therethrough; and the base plate including a central portion on which the valve element is placed, characterized in that said central portion has a thicker portion formed by swelling an upper surface of said central portion. a bottomed cylindrical body; a bottomed cylindrical plunger which is movable upward and downward while being in sliding contact with an inner circumferential surface of the body and which is biased upward by a return spring; a low pressure chamber defined in the plunger for storing an operating fluid; a high pressure chamber defined in a hollow lower interior of the body and separated from the low pressure chamber by a bottom wall of the plunger, the high pressure chamber being filled with the operating fluid; and a check valve which allows the operating fluid to flow from the low pressure chamber into the high pressure chamber when in a valve-opened state and which cuts off flow of the operating fluid from the high pressure chamber into the low pressure chamber when in a valve-closed state, the check valve including a valve orifice extending through the bottom wall of the plunger, a retainer disposed in the high pressure chamber so as to be moved upward and downward with the plunger, and a valve element which is vertically movable in the high pressure chamber between a valve-opened position at which the valve element is placed on the retainer while spaced from the valve orifice and a valve-closed position which is located higher than the valve-opened position and at which the valve element closes the valve orifice, the valve element being held in the valve-opened position by self-weight when the plunger remains still, wherein the retainer includes: a base plate on which the valve element located at the valve-opened position is placed; and a cylindrical portion extending upward from an outer circumferential edge of the base plate so as to surround the valve element; the cylindrical portion having an opening formed therethrough; and the base plate including a central portion on which the valve element is placed, characterized in that said central portion has a thicker portion formed by swelling an upper surface of said central portion. a base plate on which the valve element located at the valve-opened position is placed; and a cylindrical portion extending upward from an outer circumferential edge of the base plate so as to surround the valve element; the cylindrical portion having an opening formed therethrough; and the base plate including a central portion on which the valve element is placed, characterized in that said central portion has a thicker portion formed by swelling an upper surface of said central portion.