The present invention aims at the shortening of a main line which constitutes an
assembly line. To achieve such an object, in the present invention, an
assembly line is constituted of a main line and sub lines, and a first unit which is constituted by connecting a
clutch part and a transmission part, a second unit which constitutes a
prime mover part, a third unit to which a support body and parts such as a
handle, pedals, a
dashboard, a
fuel tank and the like which are supported on the support body are assembled, a fourth unit to which a front support body and parts such as a battery and a radiator which are supported on the front support body are assembled, a fifth unit to which steps, fenders and the like are assembled, and a sixth unit which is a hood are preliminarily assembled on sub lines and, thereafter, the above-mentioned first to sixth units are assembled together on the main line.