To enlarge an effective width of footboards in a spiral or turn-back stairs in construction of a prefabricated house by a panel construction method.
An indoor stairs are constructed by disposing a central wall panel 13 between wall panels 12 and 14 on the opposite sides disposed apart from each other by a prescribed dimension, by fixing a lower-side stairs part to the central wall panel 13 and the wall panel 12 on one side and by fixing an upper-side stairs part to the central wall panel 13 and the wall panel 14 on the other side. The central wall panel 13 is so disposed as to be offset C1 by a half t/2 of the dimension of a panel thickness toward the wall panel 14 side on the other side from the center line C0 between the opposed wall panels 12 and 14. Footboards 21 and 23 of the lower-side stairs part are laid over between the central wall panel 13 disposed in the offset state and the wall panel 12 on one side, and the lateral end parts of footboards 24 and 25 of the upper-side stairs part to be fixed between the central wall part and the wall panel 14 on the other side are placed on and supported by footboard mounting surfaces 13a which are formed in continuity in a stepped shape to and integrally with the central wall panel 13 disposed in the offset state.