To solve problems of a conventional fin as a tool for utilizing the viscosity and resistance of water in training, wherein feeling of fittingness is insufficient, wherein the length or strength of the fin itself can not be adjusted, and wherein the strength can not be adjusted even if feeling of fittingness is improved if socks are used for protecting the inside of the fin; and to provide socks for the muscle training underwater, permitting adjustment of feeling of fittingness and the strength according to the personal physical strength in the training.

A fin part 2 is attached to a part in front of the ankle of the sock 1 which can be repeatedly used underwater. The strength of the fin part 2 can be adjusted and selected by varying the width. A pocket 4 is disposed between the fin part 2 at the part in front of the ankle and the sock 1 which can be repeatedly used underwater so that a rubber resistance plate can be taken in/out of the pocket. Different kinds of the rubber resistance plates with different thicknesses in the same shape are prepared so that the strength can be adjusted and selected.