A seat supporting member designed to support, from below, a seat member in its closed position, is provided on a body frame for allowing the seat member to be attached to and detached from the seat supporting member. A rear part of the seat member and an intermediate part of the seat supporting member in a front-rear direction thereof are connected to each other via a swinging arm. The seat member is supported on a body frame while its position is variable between a closed position where the seat member allows an occupant to sit thereon while covering at least one of maintenance spaces and a housing space from above; and an open position where an upper opening of the at least one of the maintenance spaces and the housing space is widely, almost completely opened.