To provide a heat insulating box placed and self-supported on the furnace bottom of a carbonizing chamber to be used without producing an area where it is impossible to repair bottom part bricks by replacement.

The heat insulating box 11 inserted into the carbonizing chamber 3 of a coke oven to be used when the bottom bricks 3a of the carbonizing chamber 3a are repaired from the inside is used. A part or the whole of the floor surface 11a of the heat insulating box 11 is formed to be opened and to control the height level of the floor surface position in the oven height direction. After the heat insulating box 11 is inserted into the position of the carbonizing chamber where the bottom part bricks 3a are repaired, a part or the whole of the floor surface 11a of the heat insulating box 11 is opened to repair the bottom part bricks 3a of the carbonizing chamber 3. As a result, the bottom part bricks of the coke oven carbonizing chamber are smoothly repaired by replacement without the interference of the heat insulating box with the area to be repaired.