A lash adjuster includes a body formed into a bottomed cylindrical shape, a plunger movably inserted into the body and having a peripheral wall having an oil passage hole, and a partitioning member inserted into the plunger. The plunger defines a high-pressure chamber between the body and a plunger underside and includes a first threaded portion formed in an inner periphery of the peripheral wall. The partitioning member has an oil passage end located above the oil passage hole and an oil passage is defined between an outer surface of the partitioning member and the plunger peripheral wall. A low-pressure chamber is defined inside the partitioning member to reserve hydraulic fluid flowing through the oil passage hole. The hydraulic fluid is then supplied into the high-pressure chamber. The partitioning member has an outer periphery formed with a second threaded portion threadingly coupled with the first threaded portion.