Provided is a flap valve device and a gas liquid separation device provided with a flap valve. The flap valve device includes a main body having an opening, a flap for selectively closing the opening and a torsion coil spring provided between the main body and the flap for biasing the flap in an opening direction. The flap valve device further comprises a main body side bearing; a flap side bearing; a connecting member including a hinge shaft passed through the main body side bearing, the flap side bearing and the torsion coil spring, and an extension extending from an end of the hinge shaft along an outer part of the flap; and a spring engagement portion provided on the main body for engaging an end of the torsion coil spring; another end of the torsion coil spring being engaged by the extension, which is engaged on the flap.