A lash adjuster includes a body, a plunger inserted into the body and having a bottom wall with a valve hole and a peripheral wall having an oil passage hole, and a partitioning member. The plunger defines a high-pressure chamber. The partitioning member is inserted into the plunger and has an oil passage end located above the oil passage hole in an inserted state. The partitioning member has a recess defining an oil passage together with the peripheral wall. The partitioning member defines a low-pressure chamber reserving hydraulic fluid and has, together with the recess, a press-fit portion abutting against an inner periphery of the peripheral wall when the partitioning member is inserted into the plunger. Consequently, the press-fit portion has a deformed cross-sectional shape except for a circular shape when cut at a same height as the oil passage hole.