Abstract: A coupler comprising a female coupling member and a male coupling member adapted to be fitted therein for coupling air conduits of two systems, i.e., an air pressure supply system and a control system thereof, wherein the coupling members have openings of a first and a second air conduit, respectively, at respective central portions of the mating faces and annular grooves concentric with and outer than the respective openings; third and fourth air conduits open at the respective grooves; the openings of the first and the second air conduit are connected with each other; an o-ring is disposed at the connecting portion between sidewalls of the respective openings; the annular grooves of the respective coupling members are engaged with each other; a second o-ring is disposed at the engaging portion of outer walls of the respective grooves; and the engaging portion of the inner walls of the grooves are air-tightly sealed by the first o-ring.