A drive chain adjustment mechanism for simplifying the tension adjustment work of a drive chain of an all terrain vehicle. The drive chain is wound around a driven sprocket connected to the rear wheels of the vehicle. The mechanism includes a case to which the driven sprocket is coupled; a cam which moves the case in a fore and aft direction with respect to a vehicle body frame; and a bolt which is located rearward of a shaft portion of the driven sprocket, and which attaches the cam and the case to the vehicle body frame. When the case is moved in the fore and aft direction of a vehicle body, the tension of the drive chain is adjusted.