To apply propulsive force of a swimming supporting device only to one direction when holding the supporting device by hand and reciprocatingly swimming underwater.

In a handle A, the rear end of a rod-like grip part 1 has a disk-like installation base part 2. A side part of the installation base part 2 has a stopper 3, and a shaft rod 4 penetrating the side part of the base part 2 such that the shaft part 4 orthogonally intersects a central axis of the grip part 1. A pair of fins B1, B2 respectively have semicircular recessed parts 5, 6 locked to the stopper 3 in the centers of one sides, and have connection holes 7, 8, 9, 10 inserted with the shaft rod 4 on both the sides of the respective recessed parts. The fins B1, B2 are connected to the handle A in a hinge state through the shaft rod 4 penetrating the connection holes. The handle A is installed with a float 14 to obtain buoyancy in swimming.