A waste heat utilization device for an air compressor includes: a discharge path of an oil free air compressor; a circulation path along which a low boiling point medium circulates; an evaporator provided on the circulation path to heat and evaporate the low boiling point medium using the potential heat of the compressed air; and a preheater provided on an upstream side of the evaporator to preheat the low boiling point medium using the potential heat of the compressed air. A scroll type expansion machine is rotated by the low boiling point medium evaporated by the evaporator and increased in pressure, and power is generated by a power generator connected to a rotary shaft of the scroll type expansion machine. The low boiling point medium discharged from the scroll type expansion machine is then cooled and condensed by a condenser.