The swing arm device is divided into an arm body and a rear housing body. The arm body has a front end pivot shaft, a drive shaft case, a leg portion and a brake case provided behind the drive shaft case and is an integral aluminum casting. The rear housing body has a gear case, a left case portion and a rear axle case and is an integral aluminum casting. The dividing plane is almost perpendicular to the shaft axis of a drive shaft. The arm body and the rear housing body are separatably fastened together with a bolt. Therefore, in the swing arm device, a small number of parts are required, a welding work takes less labor and time, a necessary rigidity can easily be kept and an assembling work can easily be carried out.