PURPOSE: To enable strong fastening in a device wherein a shaft and a boss are fastened through a key fastening structure by welding a thin plate onto a key main body while forming a sealed space, sealing pressure medium in the space by means of a pressure supplying means, and thereby expanding the thin plate.

CONSTITUTION: A key main body 2 being cross sectionally T-shaped has a lower narrow portion 2b which is fitted to a key groove 6 of a shaft 4. A circumference of a thin plate 8 is fixed to an upper surface of a wide portion 2a of the key main body 2, and a sealed space 10 is formed between the thin plate 8 and the upper surface of the wide portion 2a. Pressure medium such as oil is sealed in the space 10. A tube portion 12 is formed on one end of the key main body 2. An inner diametral portion of the tube portion 12 is communicated with the space 10 through a hole 14. A pressure screw 16 is screwed with a thread hole formed on the inner diametral portion of the tube portion 12. By fastening the pressure screw 16, pressure is applied to the pressure medium inside the space 10, and the thin plate 8 is expanded so as to be strongly pressurized against an inner surface of a key groove 20 on the side of a boss.