A chain tensioner comprising: a housing in which a cylinder chamber is defined such that one end of the cylinder chamber opens; a tubular plunger which includes a bottom, and an internal thread on an inner portion of the tubular plunger, and which is inserted in the cylinder chamber so as to be protrudable from the cylinder chamber; a screw rod including an external thread on an outer periphery of the screw rod, and disposed inside of the tubular plunger; a return spring disposed between the screw rod and the tubular plunger, and configured to bias the tubular plunger in a direction in which the tubular plunger protrudes from the cylinder chamber; a pressure chamber defined in a back portion of the tubular plunger within the cylinder chamber; an oil supply passage communicating with the pressure chamber; and a check valve configured to close the oil supply passage when a pressure in the pressure chamber is higher than a pressure in the oil supply passage, wherein the internal thread and the external thread have: a first plunger-pushed-in-side flank and a second plunger-pushed-in-side flank, respectively, that are configured to abut against each other when the tubular plunger is pushed into the cylinder chamber; and a first plunger-protruding-side flank and a second plunger-protruding-side flank, respectively, that are configured to abut against each other when the tubular plunger protrudes from the cylinder chamber, wherein the internal thread is serrated such that a flank angle (α1) of the first plunger-pushed-in-side flank of the internal thread is larger than a flank angle (α2) of the first plunger-protruding-side flank of the internal thread, wherein the external thread is serrated such that a flank angle (α1) of the second plunger-pushed-in-side flank of the external thread is larger than a flank angle (α2) of the second plunger-protruding-side flank of the external thread, and wherein the serrated internal and external threads (15 and 17) are threadedly engaged with each other, characterized in that the internal thread of the tubular plunger has a pitch (a) smaller than a pitch (b) of the external thread of the screw rod. a housing in which a cylinder chamber is defined such that one end of the cylinder chamber opens; a tubular plunger which includes a bottom, and an internal thread on an inner portion of the tubular plunger, and which is inserted in the cylinder chamber so as to be protrudable from the cylinder chamber; a screw rod including an external thread on an outer periphery of the screw rod, and disposed inside of the tubular plunger; a return spring disposed between the screw rod and the tubular plunger, and configured to bias the tubular plunger in a direction in which the tubular plunger protrudes from the cylinder chamber; a pressure chamber defined in a back portion of the tubular plunger within the cylinder chamber; an oil supply passage communicating with the pressure chamber; and a check valve configured to close the oil supply passage when a pressure in the pressure chamber is higher than a pressure in the oil supply passage, wherein the internal thread and the external thread have: a first plunger-pushed-in-side flank and a second plunger-pushed-in-side flank, respectively, that are configured to abut against each other when the tubular plunger is pushed into the cylinder chamber; and a first plunger-protruding-side flank and a second plunger-protruding-side flank, respectively, that are configured to abut against each other when the tubular plunger protrudes from the cylinder chamber, wherein the internal thread is serrated such that a flank angle (α1) of the first plunger-pushed-in-side flank of the internal thread is larger than a flank angle (α2) of the first plunger-protruding-side flank of the internal thread, wherein the external thread is serrated such that a flank angle (α1) of the second plunger-pushed-in-side flank of the external thread is larger than a flank angle (α2) of the second plunger-protruding-side flank of the external thread, and wherein the serrated internal and external threads (15 and 17) are threadedly engaged with each other, characterized in that the internal thread of the tubular plunger has a pitch (a) smaller than a pitch (b) of the external thread of the screw rod.