A lash adjuster includes a cylindrical body, a cylindrical plunger, a low pressure chamber defined in the plunger, a high pressure chamber defined in a hollow lower interior of the body and filled with an operating fluid, a check valve allowing the operating fluid to flow into the high pressure chamber when in a valve-opened state and cutting off flow of the operating fluid from the high pressure chamber into the low pressure chamber when in a valve-closed state. A retainer of the check valve includes a base plate and a cylindrical portion extending upward from an outer circumferential edge of the base plate so as to surround the valve element. The base plate includes a central portion on which the valve element is placed and which has an upper surface formed with a swollen portion. The cylindrical portion has a plurality of openings formed through it circumferentially equiangularly.