To provide a socket for supplying compressed air, which socket can supply the compressed air regulated so as to have the air pressure optimum to both of a high-pressure pneumatic instrument driven by high-pressure compressed air and an atmospheric pressure pneumatic instrument driven by atmospheric pressure compressed air.

The tip end portion 60b of a high-pressure dedicated plug 60 projects as a small-diameter cylindrical portion. A plurality of fluid passages 60c are formed on the side surface of the cylindrical portion. When a high-pressure dedicated plug 60 has been inserted into a plug connecting port 29, the tip end portion 60b of the high-pressure dedicated plug 60 is brought into contact with a forced valve opening member 16, and pushes the forced valve opening member 16 so as to move the forced valve opening member 16 toward a partition wall 3b. As a result, the piston 11 of a pressure control valve is pushed and fixed to the partition wall 3b, and the projecting portion 11b of the piston 11 of the pressure control valve makes a valve body 9 separate from an annular projecting portion 3f. Thus, the valve body 9 is fixed in the valve opening state.