A lash adjuster includes a plunger having a bottom wall with a valve hole and a peripheral wall having an oil passage hole and an inner periphery formed with a recessed groove, the plunger defining a high-pressure chamber between the bottom wall and a body, and a partitioning member having an oil passage end located above the oil passage hole. The partitioning member has an oil-passage defining portion located opposite the recessed groove and defining an oil passage between itself and a groove face of the recessed groove. The partitioning member defines a low-pressure chamber reserving a hydraulic fluid flowing through the oil passage hole, the oil passage and the oil passage end. The low-pressure chamber causes the reserved hydraulic fluid to flow through the valve hole into the high-pressure chamber. The partitioning member is formed into a cylindrical shape and extends in an up-down direction without any stepped part.