To provide a forming device to form a plate for building use by executing the plate for building use of horizontal fitting type which is formed in a curved shape on the horizontal plane along the longitudinal direction and an external surrounding body with excellent appearance by the plate for building use.

An inner circumferential riser part 2 is formed from one edge side on the inner circumferential side in the width direction of a main plate, and a lower holding part 3 is formed from an upper end of the inner circumferential side riser part 2. An outer circumferential side hanging part 4 is formed from an edge side on the other outer circumferential side in the width direction of the main plate 1, and an upper hooking part 5 capable of being hooked and tightened to the lower hooking part 3 is formed from an upper end of the outer circumferential side hanging part 4. A main plate corrugated surface 1w is formed on the main plate 1 so that the inner circumferential side riser part 2 is shorter than the outer circumferential side hanging part 4, and an inner circumferential side corrugated surface 2w is formed on the inner circumferential side riser part 2 respectively.