To provide an outdoor panel structure of a building with a waterproof sheet welded in a joint part between adjacent waterproof panels, which, even when solvent steam is generated, can suppresses the occurrence of inconvenience such as dissolution or deformation of a heat insulating material caused by it.

A roof panel 10 is provided with a sheathing roof board 11, the heat insulating material 12, a plurality of noncombustible steel plates 13, and the belt-like waterproof sheet 15. The heat insulating material 12 is installed on the sheathing roof board 11, and the noncombustible steel plates 13 are arranged over the heat insulating material 12 in the same plane. The belt-like waterproof sheet 15 is arranged along the joint part between the adjacent noncombustible steel plates 13. The two noncombustible steel plates 13 adjacent to each other are arranged across a predetermined space C, and the waterproof sheet 15 is joined on the panel surface by welding in such a state as to overlap both of the noncombustible steel plates 13. A joint tape 17 having solvent resistance is attained to the joint part of the noncombustible steel plates 13 across the adjacent noncombustible steel plates 13.