A fuel tank check valve, which can prevent one portion of a coil portion of a torsion coil spring from being stuck on a stepped portion of a pivot includes a cylindrical member defining a flow channel for a fuel fluid; a flap axially supported on the cylindrical member through a pivot; and a torsion coil spring urging the flap in a closing direction. The pivot includes a large diameter portion around which a coil portion of the torsion coil spring is wound; and a first small diameter portion and a second small diameter portion coaxial with each other, and projecting into a shaft hole provided in either one of the cylindrical member or the flap. The first small diameter portion adjacent to the large diameter portion is off-centered to a side where the torsion coil spring presses against the pivot relative to the large diameter portion.