Abstract

The present invention is directed to varying the oil pressure in the chamber of a hydraulic tensioner at multi-stages and preventing an increase of the chain friction at various driving conditions. A hydraulic tensioner (1) includes a housing (2) that has a plunger bore (2a) to slidably receive the plunger (3) and a spool bore (2b) to slidably receive the spool (4). The housing (2) further has an inner relief aperture (22) to provide a connection between the plunger bore and the spool bore and outside relief apertures (23a, 23b) to provide a connection between the spool bore and the outside of the housing. The entire opening areas of the inside and outside relief apertures (22, 23a, 23b) provide a connection between the plunger bore and the outside of the housing through the spool bore (2b) and vary at least at two stages in accordance with the travel of the spool (4).