A foot cover includes a pair of laterally spaced surface sections, first engagement portions on the insides of the lateral surface sections and configured to be engaged with a front side of a leg bracket, a second engagement portion at a rear side of the foot cover and configured to engage a rear side of the leg bracket, and an urging rib that urges the foot cover toward the rear side thereof. The first engagement portions have a fore-and-aft restricting surface that restricts the foot cover from being displaced rearwardly when mounted and a first inclination surface that abuts an associated front end portion of the leg bracket to restrict deformation of the associated lateral surface section into a separating direction of the lateral surface sections.