PURPOSE: To dispense with the manual air bleeding work of a mat by adding the function of discharging air from a mat with air inside to the outside.

CONSTITUTION: When air supply is selected by the rotation of the change-over lever of a change-over element 1, a shielding plate 15 covers a discharge port 12a to the outside and a suction nipple 13b from a mat so as to supply air to the mat. When air discharge from the mat to the outside is selected, the shielding plate 15 covers a discharge nipple 13a to the mat and a suction port 12b from the outside so as to suck air into a suction chamber 1b from the mat through the suction nipple 13b. This air is fed into the discharge chamber 1a of the change-over element 1 through a filter case, a tank, and the like and discharged outside from the discharge port 12a.