To provide a foreign substance intrusion preventing structure of an auxiliary machine drive belt capable of more reliably preventing foreign substances from intruding, in an engagement position where the auxiliary machine drive belt is engaged with a pulley.

A foreign substance intrusion preventing portion 10 is provided in parallel in a direction away from an engagement position P1, and has a plurality of partitions 11, 12 erected between the crank pulley 3 and the auxiliary machine drive belt 6, respectively, with clearances g1, g1', g2, g2' created with respect to a crank pulley 3 and an auxiliary machine drive belt 6.