To provide a fireproof joint structure which is obtained by imparting fire resistance to a joint portion defined by both side surfaces of two or more fireproof panels set on a floor surface, a roof, a ceiling, etc. of a building, and excellent in working efficiency.

The fireproof joint structure is formed of the two or more fireproof panels horizontally set at least on one selected from the group consisting of the floor surface, the roof, and the ceiling of the building, and a T-shaped joiner set at the joint portion. The T-shaped joiner includes a holding portion formed in parallel with an upper surface of the fireproof panel, and a joint insertion portion formed perpendicularly to the holding portion, and at least contains a thermo-expandable fireproof material. The holding portion of the T-shaped joiner is held by the upper surface of one of the fireproof panel and the upper surface of the other fireproof panel, and the joint insertion portion of the T-shaped joiner is inserted into the joint portion defined by the side surface of the one fireproof panel and the side surface of the other fireproof panel.