To provide a tool fixing device for the main spindle of a machine tool, capable of generating strong energizing force by means of a fluid-operated cylinder by providing the fluid-operated cylinder with a pressure intensifying function to generate a high hydraulic pressure by pressurizing oil in a hydraulic cylinder by means of the pressure of a gas in a gas spring.

The hydraulic cylinder 5 of this tool fixing device 1 is equipped with the oil-filled type hydraulic cylinder 20 hermetically filled with the oil L and the gas-filled type gas spring 21 hermetically filled with the compressed gas G. The oil L in an oil chamber 31 of the hydraulic cylinder 20 is pressurized by the gas spring 21 to a pressure higher than the pressure of the gas in a gas operating chamber 41 of the gas spring 21, thereby generating strong energizing force by means of an output piston member 32 of the hydraulic cylinder 20.