A guiding roller unit has a holder to be mounted on a mount, and a guiding roller rotatably supported by the holder. The holder includes a base, two bearings and widening preventing plates attached respectively to the bearing. The base, the bearings and the widening preventing parts are integral to each other. The base is mounted on the mount. The bearings stand from opposite ends of the base and rotatably support the roller. The widening preventing plates extend in directions opposite to facing directions of the bearings. The holder enables the roller to be attached and detached with leading ends of the bearings elastically widened with respect to base ends thereof connected with the base while being detached from the mount, whereas it prohibits widening of the leading ends of the bearings by contacting the mount in response to forces acting in widening directions while being mounted on the mount.