A tensioner arm comprises a tool-engagement portion for engaging with a tensioner-arm rotation tool, and a jig-holding portion which has a through hole for accepting a tensioner-arm fixation bar jig being inserted thereinto from above at a portion of the tensioner arm extending upward from the base end portion. A bearing case comprises a contact face portion which is provided at a face located below the jig-holding portion of the bearing case so as to accept the bar jig being inserted into the through hole of the tensioner arm, and an engagement stair portion which is provided at the contact face portion so as to engage with the bar jig such that move of the tensioner arm toward the belt by the biasing mechanism is restricted. A belt tensioner which enables an easy belt exchanging even by a single worker without detaching surrounding components of the engine can be provided.