To provide assist devices making a propulsion force apply only in one direction and having buoyancy when swimming by reciprocating the swimming assist devices in hands.

A handle A is provided with a mount base 2 in the rear end of a grip part 1, and the mount base is provided with stoppers 3 and 4 in its both sides and bearings 5, 6, 7 and 8 in its rear parts. The mount base is engaged with a pair of gears 15 and 16 comprising hubs 9, 10, 11 and 12 in the both ends and through-holes 13 and 14 in the central part, attached with a pair of parallel shaft rods 17 and 18, which orthogonally cross with the central line of the grip part, via the hubs by the bearings and a mount member 19, and provided with a stopper 20 and an energizing spring 27 in one face of the mount member. Fins B1 and B2 have planar square shape and are provided with recesses 21 and 22 having a shape of fitting to the mount base and insertion holes 23, 24, 25 and 26. The hand fin is rotationally connected to the handle via the shaft rod. The hand fin with a float is provided with the float 30 with a floating body 29 attached to a bridge 28 in the upper part of the grip.