A ratchet-type tensioner, comprising: a housing; a plunger supported by the housing for movement in opposite advancing and setback directions, the plunger having rack teeth provided thereon, and being arranged to advance with respect to the housing along the advancing direction for applying tension to a traveling transmission medium engaged with rotating members; and a ratchet mechanism capable of restricting the plunger from setting back due to a reaction force acting in a setback direction from the transmission medium; said ratchet mechanism comprising: a ratchet-receiving hole provided within the housing; a ratchet element slidable in the ratchet-receiving hole and movable in a sliding direction transverse to the advancing and setback directions; ratchet teeth on the ratchet element engageable with the rack teeth of the plunger; and a ratchet-biasing means for biasing the ratchet element in an engaging direction along the sliding direction so that the ratchet teeth engage with the rack teeth; said ratchet mechanism being capable of restricting the plunger from being set back by a reaction force acting in a setback direction from the transmission medium; said rack teeth having a concave-convex form and being composed of stop surfaces facing in the setback direction and sliding surfaces facing in the advance direction, said sliding surfaces being inclined relative to the sliding direction and facing in a disengaging direction opposite from the engaging direction; each of said sliding surfaces having an entry starting location which the ratchet teeth first engage when starting to enter the rack teeth, and an entry ending location where the engagement of the ratchet teeth with the rack teeth ends; each said sliding surface being a curved surface, all of said curved surface, between said entry starting location and said entry ending location of said sliding surface, bulging in the advancing direction of the plunger and in the disengaging direction of the ratchet element from an imaginary reference plane intersecting said entry starting location and said entry ending location of said sliding surface and perpendicular to another imaginary plane to which the advancing and setback direction of the plunger and the sliding direction of the ratchet element are parallel; and at least a part of said curve midway along said curve between the entry starting location and the entry ending location is convex.