PURPOSE: To enable a rotary machine to hold a fixed rotary speed even for a change in a load and original pressure of air, by providing a tachometer generator and an air flow regulating valve, in the case of a rotary speed control device for the pneumatically driven rotary machine of an air grinder or the like.

CONSTITUTION: A control device, connecting a speedometer generator (tachometer generator) 2 with a pneumatically driven rotary machine 1 of air grinder or the like, provides an air flow regulating valve 3 between an air supply source 4 and the rotary machine 1. This air flow regulating valve 3, increasing its opening while a flow of air when a speed of the rotary machine 1 is lower than the rated speed with said speed of the machine in a control start speed or more while decreasing the opening and the flow of air when the speed of the machine increases higher than the rated speed, controls the flow of air, supplied to the rotary machine 1 by an output signal S of the tachometer generator 2, so as to hold the rated speed. In this way, the rotary speed control device, corresponding to a change in a load and the original pressure while eliminating any necessity for an external power supply, is formed in small size and light weight.