PURPOSE: To prevent icing of the air vent pipe of a double-bottomed tank just below the frozen cargo hold of a refrigerated ship by forming one part of a horizontal bulkhead into a wooden trunk, and passing a pipe through the trunk, and packing urethane foam around the pipe, and installing an insulating body at that portion of the pipe which passes through a steel deck.

CONSTITUTION: The air vent pipe 2 and the sounding pipe 3 of a double- bottomed tank comprises one part of a horizontal bulkhead 13 led to the inside of a wooden trunk 6 formed separate from a cargo hold by a wooden wall 5 and urethane foam is packed around the pipes 2, 3 inside the wooden trunk 6. An insulating body 10 is installed at that portion 9 of each pipe which passes through a steel deck 8. By this constitution heat transmission between each pipe and a ship body is weakened so as to prevent icing of each pipe.