least one ratchet tooth from the rack teeth against the biasing force of the plunger biasing spring and a second oil supply passage for introducing oil under pressure into the second high pressure oil chamber and the oil supply passages are isolated from each other sufficiently that the oil pressures in the oil supply passages can be different the ratchet tensioner blocks backlash of the plunger due to force receiving from the timing chain on starting the engine when the engine has been inoperative for an extended interval and generates hydraulic pressure stably within the second high pressure oil chamber to achieve quick forward and backward movement of the plunger smoothly oil supplied from an oil pump can be introduced directly into the first oil supply passage in the housing or can be introduced through an oil reservoir formed on a back of the housing for temporarily storing oil supplied from the oil pump before introducing it into the first oil supply passage the check valve unit used in the ratchet tensioner of the invention can be any of various check valve types as long as it is assembled at the bottom of the plunger accommodating hole and blocks reverse flow of the oil from the high pressure oil chamber to the first oil supply passage for example the check valve unit may have a ball seat that communicates with the first oil supply passage and supplies oil under pressure to the high pressure oil chamber a check ball that faces a seating surface of the ball seat a ball biasing spring for pressing.