PURPOSE: To cool the contents of a blast furnace in a short time without damaging the bricks in the furnace bottom by blowing liquid nitrogen through an injecting pipe inserted into the furnace through its circumferential wall into the remaining contents in the blast furnace after reduction of the stock line and blowing out.

CONSTITUTION: After reduction of the stock line and blowing out of a blast furnace 1, a furnace bottom 12 is bored to open horizontal holes 16 circumferentially in plural places and the molten metal remaining below the level of a tap hole 15 is removed to the outside of the furnace; thereafter the holes 16 are closed. An injecting pipe 17 for liquid nitrogen consisting of double pipes interposed with a heat insulating material layer is inserted through the hole 15 into the furnace 1. The preceding end part of the pipe 17 is inserted to a required length in the residual materials in the furnace and partly gasified liquid nitrogen is injected into the residual materials from the root end part to cool said materials. If necessary, the wall 11 near the bottom 12 is bored to open a through-hole and said pipe 17 may be inserted into the residual materials in the furnace through said hole.