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 inserted into the plunger to be fixed therein and formed into a tubular shape, 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; the hydraulic fluid is further caused to fill the high-pressure chamber through the valve hole whereby the plunger is moved up and down according to a hydraulic pressure in the high-pressure chamber; the plunger has an inner periphery formed with a recessed groove; the partitioning member has an outer periphery formed with an oilpassage defining portion which is located opposite the recessed groove; and the oil passage is defined between a groove face of the recessed groove and the oil-passage defining 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 inserted into the plunger to be fixed therein and formed into a tubular shape, 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; the hydraulic fluid is further caused to fill the high-pressure chamber through the valve hole whereby the plunger is moved up and down according to a hydraulic pressure in the high-pressure chamber; the plunger has an inner periphery formed with a recessed groove; the partitioning member has an outer periphery formed with an oilpassage defining portion which is located opposite the recessed groove; and the oil passage is defined between a groove face of the recessed groove and the oil-passage defining 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; the hydraulic fluid is further caused to fill the high-pressure chamber through the valve hole whereby the plunger is moved up and down according to a hydraulic pressure in the high-pressure chamber; the plunger has an inner periphery formed with a recessed groove; the partitioning member has an outer periphery formed with an oilpassage defining portion which is located opposite the recessed groove; and the oil passage is defined between a groove face of the recessed groove and the oil-passage defining portion.