pulley 5 then successively drives a pulley for power steering pump 6 a pulley for secondary air pump 7 an alternator pulley 8 and a pulley for fan water pump 9 the engine fan is usually mounted on the water pump shaft this serpentine drive system is distinguished by the fact that for example the pulley for fan water pump 9 disposed at the center of the group of the auxiliary machinery is driven by the back surface of the belt 4 a belt tension control device 11 having an idle pulley 10 may be disposed on the engine body 1 so as to adjust the tension of the belt 4 as required t a t b t g designate the tension between each adjacent two of said auxiliary machines the magnitude of the tension t a is almost independent of the order of arrangement of the auxiliary machinery except when the total driven torque of the auxiliary machinery is changed on the other hand the magnitude of the tensions t b t c may considerably vary according to their order of arrangement and the reduction of the tensions t b and t c below t a are considerably large that is to say when an auxiliary machine having a large driven torque is disposed closest to the driving pulley in the direction of movement of the belt the magnitude of the tensions of the successive auxiliary machines become smaller thus the larger the magnitude of the driven torque of the air conditioner the smaller the magnitude of the tensions t b t c since the driven torques of the auxiliary machinery 6 7 8 and 9 are relatively small the tensions t d t e t f and t g are small and the changes in the tensions t.