To provide a speed control method reducing power consumption required for operation of an oil-free compressor as much as possible.

The corresponding relationship between the change in suction air temperature Ts sucked into a compressor body 10 and the change in rotation speed where the compressor body 10 discharges compressed air of a set maximum allowable temperature Tdmax, is acquired in advance. A memory means 44 stores the corresponding relationship. After a temperature sensor or the like measures the suction air temperature Ts sucked into the compressor body 10, the rotation speed for generating the maximum allowable temperature Tdmax by the compressor body 10 is calculated from the measured suction temperature Ts based on the corresponding relationship stored in the memory means 44, and the calculated rotation speed is output to an inverter 31 and is set as an unload rotation speed Nmin.