A lash adjuster including: a body formed into a bottomed cylindrical shape; a plunger which is inserted into the body so as to be movable up and down and has a bottom wall and a peripheral wall standing from an outer periphery of the bottom wall, so that the plunger is formed into a bottomed cylindrical shape; and a partitioning member having a tubular shape and inserted into the plunger, wherein: the plunger has an oil passage hole formed through the peripheral wall thereof and the partitioning member has an oil passage end located above the oil passage hole in a state where the partitioning member is inserted into the plunger; a high-pressure chamber is defined between the bottom wall of the plunger and the body; a low-pressure chamber is defined inside the partitioning member in an interior of the plunger; the bottom wall of the plunger has a valve hole capable of communicating with the high-pressure chamber and the low-pressure chamber; an oil passage is defined outside the partitioning member in the interior of the plunger; a hydraulic fluid is reserved through the oil passage hole, the oil passage and the oil passage end into the low-pressure chamber; and the hydraulic fluid is further caused to fill the high-pressure chamber through the valve hole, characterized in that the peripheral wall of the plunger has an inner peripheral surface provided with a plunger side threaded portion and the partitioning member has an outer peripheral surface provided with a partitioning member side threaded portion configured to be threadingly coupled with the plunger side threaded portion. a body formed into a bottomed cylindrical shape; a plunger which is inserted into the body so as to be movable up and down and has a bottom wall and a peripheral wall standing from an outer periphery of the bottom wall, so that the plunger is formed into a bottomed cylindrical shape; and a partitioning member having a tubular shape and inserted into the plunger, wherein: the plunger has an oil passage hole formed through the peripheral wall thereof and the partitioning member has an oil passage end located above the oil passage hole in a state where the partitioning member is inserted into the plunger; a high-pressure chamber is defined between the bottom wall of the plunger and the body; a low-pressure chamber is defined inside the partitioning member in an interior of the plunger; the bottom wall of the plunger has a valve hole capable of communicating with the high-pressure chamber and the low-pressure chamber; an oil passage is defined outside the partitioning member in the interior of the plunger; a hydraulic fluid is reserved through the oil passage hole, the oil passage and the oil passage end into the low-pressure chamber; and the hydraulic fluid is further caused to fill the high-pressure chamber through the valve hole, characterized in that the peripheral wall of the plunger has an inner peripheral surface provided with a plunger side threaded portion and the partitioning member has an outer peripheral surface provided with a partitioning member side threaded portion configured to be threadingly coupled with the plunger side threaded portion. the plunger has an oil passage hole formed through the peripheral wall thereof and the partitioning member has an oil passage end located above the oil passage hole in a state where the partitioning member is inserted into the plunger; a high-pressure chamber is defined between the bottom wall of the plunger and the body; a low-pressure chamber is defined inside the partitioning member in an interior of the plunger; the bottom wall of the plunger has a valve hole capable of communicating with the high-pressure chamber and the low-pressure chamber; an oil passage is defined outside the partitioning member in the interior of the plunger; a hydraulic fluid is reserved through the oil passage hole, the oil passage and the oil passage end into the low-pressure chamber; and the hydraulic fluid is further caused to fill the high-pressure chamber through the valve hole, characterized in that the peripheral wall of the plunger has an inner peripheral surface provided with a plunger side threaded portion and the partitioning member has an outer peripheral surface provided with a partitioning member side threaded portion configured to be threadingly coupled with the plunger side threaded portion.