To provide an auxiliary drive device capable of suppressing generation of excessive vibrations on an auxiliary belt while avoiding worsening of fuel consumption and an increase of manufacturing cost, and thereby, capable of improving merchantability.

An auxiliary drive device 1 for driving a plurality of auxiliaries such as AC power generator 11 with a power of internal combustion engine 3 includes: a crank pulley 5; four pulleys 7 to 10 respectively connected to four auxiliaries; and the endless auxiliary belt 4 wound over the crank pulley 5 and the pulleys 7 to 10. A power generator pulley 8 is configured such that power transmission ability between the power generator pulley 8 and the auxiliary belt 4 becomes smaller than that of the crank pulley 5.