To prevent breakage of a machining tool in a way that the number of revolutions does not exceed a maximum allowable value even when a high air pressure is supplied.

In the displacement position of a member 32 for speed regulation when a rotor attains the allowable number of revolutions, an amount of feed air passing through the section of an air introduction passage 12 consisting of the member 32 for speed regulation and the venturi part 48 of a main body 10 is limited. By this limit air amount, the number of revolutions of the rotor is limited, and is prevented from exceeding the allowable number of revolutions of a machining tool. A space 62 is formed between the tip closing part 52 of the member 32 and the tip end surface of a shaft 30 for speed regulation, and one of through small holes 58 formed in the protrusion part 54 of the tip end closing part 52 of the shaft 30 for speed regulation is communicated with the space 62 and the other is communicated with the air introduction passage 12. Expansion and contraction vibration in an axial direction of the member 32 is prevented from occurring by the resistance force of air inputting to and outputting from the space 62 after the passage of it through the through-small hole 58.