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 (13, 13A) inserted into the plunger, wherein: the plunger has an oil passage hole formed through the peripheral wall thereof and the partitioning member (13, 13A) has an oil passage end located above the oil passage hole in a state where the partitioning member (13, 13A) 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 (13, 13A) 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 (13, 13A) 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; the partitioning member (13, 13A) has a plate-like baffle located opposite the oil passage hole in an inserted state thereof into the plunger; the plunger has an interior divided into two spaces by the baffle, the oil passage hole being located at one space side; the oil passage is defined at said one space side; and the low-pressure chamber is defined at the other space side, characterized in that the partitioning member (13, 13A) has support portions (38, 38A) which partially extend outward from both widthwise ends of the baffle respectively and abut on an inner periphery of the peripheral wall of the plunger, thereby retaining the baffle in a standing position. 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 (13, 13A) inserted into the plunger, wherein: the plunger has an oil passage hole formed through the peripheral wall thereof and the partitioning member (13, 13A) has an oil passage end located above the oil passage hole in a state where the partitioning member (13, 13A) 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 (13, 13A) 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 (13, 13A) 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; the partitioning member (13, 13A) has a plate-like baffle located opposite the oil passage hole in an inserted state thereof into the plunger; the plunger has an interior divided into two spaces by the baffle, the oil passage hole being located at one space side; the oil passage is defined at said one space side; and the low-pressure chamber is defined at the other space side, characterized in that the partitioning member (13, 13A) has support portions (38, 38A) which partially extend outward from both widthwise ends of the baffle respectively and abut on an inner periphery of the peripheral wall of the plunger, thereby retaining the baffle in a standing position. the plunger has an oil passage hole formed through the peripheral wall thereof and the partitioning member (13, 13A) has an oil passage end located above the oil passage hole in a state where the partitioning member (13, 13A) 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 (13, 13A) 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 (13, 13A) 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; the partitioning member (13, 13A) has a plate-like baffle located opposite the oil passage hole in an inserted state thereof into the plunger; the plunger has an interior divided into two spaces by the baffle, the oil passage hole being located at one space side; the oil passage is defined at said one space side; and the low-pressure chamber is defined at the other space side, characterized in that the partitioning member (13, 13A) has support portions (38, 38A) which partially extend outward from both widthwise ends of the baffle respectively and abut on an inner periphery of the peripheral wall of the plunger, thereby retaining the baffle in a standing position.