To carry out the post-treatment of a power supply cable easily by a method wherein a cap which is driven into the bottom end part of the vertical cylinder of a socket main part and made of synthetic resin and nails which are inserted through the nail holes of the socket main part and driven into a floor frame and made of synthetic resin are provided.

A through-hole 1 and a punch-through cover 9 are formed in parallel in the top plate 2 of the vertical cylinder 3 of the synthetic resin socket main part 6 of a power supply cable socket (a). The inner diameter of the lower end part of the vertical cylinder 3 is a little enlarged and protruding parts 4 are formed integrally on both the outer sides of the lower part of the vertical cylinder 3. Vertical nail holes 5 are formed in both the protruding parts 4. Synthetic resin nails 8 are inserted into the nail holes 5 and driven into a floor frame and, further, a synthetic resin cap 7 which is tapered 11 so as to reduce the diameter of the upper part is driven into the bottom end part of the vertical cylinder 3 of the socket main part 6. With this constitution, the post-treatment of a power supply cable can be carried out easily and simply without spoiling the appearance.