To supply a sufficient quantity of compressed air, without a pressure drop, to a compressed air tool, etc. driven by an air motor.

This air plug is an air plug 15 of a fluid coupling constituted of a socket for connecting the compressed air tool provided with an air chamber 8 storing compressed air supplied from a compressed air supply source and having capacity set to be ≤90 cc to a compressed air supply source and the air plug 15. An outer diameter of a sealing part 19 of the air plug 15 fitted into the socket is formed to be ≤7.1 mm. An air flow passage 20 formed at the center of the sealing part 19 on the air plug 15 side into which compressed air flows from the socket side is formed to have flow passage cross section in a space corresponding to a hole having a diameter ≥3 mm. The air flow passage 20 is opened by the same flow passage cross section at an end face of the sealing part 19.