A chain tensioner comprising: a tubular cylinder having one end thereof open, and the other end thereof closed; a tubular plunger axially slidably supported by an inner periphery of the cylinder, the plunger having an open end located inside the cylinder, and a closed end protruding out of the cylinder; a return spring biasing the plunger in a direction in which the plunger protrudes beyond the one end of the cylinder; a pressure chamber defined in the cylinder such that a volume of the pressure chamber changes with axial movement of the plunger; a reservoir chamber defined within the plunger; a check valve that allows only a flow of oil from the reservoir chamber toward the pressure chamber; a leakage gap between an outer periphery of the plunger and an inner periphery of the cylinder; a communication path that allows communication between the leakage gap and an interior of the plunger; an oil supply path connecting inside of the cylinder to outside of the cylinder; and a tubular separating member rising from the check valve toward the one end, the separating member having an opening that opens into the reservoir chamber, at a location closer to the one end than is the oil supply path wherein the check valve comprises: a valve seat formed with a valve hole connecting the reservoir chamber with the pressure chamber; and a valve element configured to open and close the valve hole, characterized in that the separating member includes a connecting portion (43, 46) at the other end of the separating member, the connecting portion (43, 46) being fixed in position by being sandwiched between the plunger and the valve seat, so that the opening of the separating member at the one end thereof does not come into contact with an inner end surface of the plunger at the one end thereof. a tubular cylinder having one end thereof open, and the other end thereof closed; a tubular plunger axially slidably supported by an inner periphery of the cylinder, the plunger having an open end located inside the cylinder, and a closed end protruding out of the cylinder; a return spring biasing the plunger in a direction in which the plunger protrudes beyond the one end of the cylinder; a pressure chamber defined in the cylinder such that a volume of the pressure chamber changes with axial movement of the plunger; a reservoir chamber defined within the plunger; a check valve that allows only a flow of oil from the reservoir chamber toward the pressure chamber; a leakage gap between an outer periphery of the plunger and an inner periphery of the cylinder; a communication path that allows communication between the leakage gap and an interior of the plunger; an oil supply path connecting inside of the cylinder to outside of the cylinder; and a tubular separating member rising from the check valve toward the one end, the separating member having an opening that opens into the reservoir chamber, at a location closer to the one end than is the oil supply path a communication path that allows communication between the leakage gap and an interior of the plunger; an oil supply path connecting inside of the cylinder to outside of the cylinder; and a tubular separating member rising from the check valve toward the one end, the separating member having an opening that opens into the reservoir chamber, at a location closer to the one end than is the oil supply path wherein the check valve comprises: a valve seat formed with a valve hole connecting the reservoir chamber with the pressure chamber; and a valve element configured to open and close the valve hole, a valve seat formed with a valve hole connecting the reservoir chamber with the pressure chamber; and a valve element configured to open and close the valve hole, characterized in that the separating member includes a connecting portion (43, 46) at the other end of the separating member, the connecting portion (43, 46) being fixed in position by being sandwiched between the plunger and the valve seat, so that the opening of the separating member at the one end thereof does not come into contact with an inner end surface of the plunger at the one end thereof.