PURPOSE: To allow efficient cooling of heat receiving face by constituting a cooling pipe of inner and outer pipes, setting the pressure in the inner pipe higher than that in the outer pipe, and ejecting cooling water to a thermal load part through a hole made at a part of the inner pipe.

CONSTITUTION: The cooling pipe is constituted of inner and outer pipes 2, 1 and a hole 3 is made through the inner pipe 2 at a part subjected to thermal load. Cooling water passes from inlet header pipes 16, 16' through the inner and outer pipes 2, 1 to cool a heat receiving plate 13 thence flows to outlet header pipes 15, 15' where the cooling water is pressurized by means of pumps 17, 17' and circulated through channels 18, 18'. Since the pressure in the inner pipe is set higher than that in the outer pipe 1, cooling water flowing through the inner pipe 2 is sprayed through the holes 3 toward the wall of the outer pipe 1 which is thereby cooled concentrically. Cooling water thus sprayed is then combined with the cooling water flowing through the outer pipe 1. Since the cooling water flowing through the inner pipe 2 is sprayed toward a part susceptible to high thermal load, that part is cooled constantly with fresh cooling water to enhance thermal conductivity thus preventing boiling.