To excellently form a projection and a recess for fitting, at both ends of a metallic plate by avoiding deformation at both ends, in forming a rugged face through roll forming of projected and recessed parts while the metallic plate is fed without interruption for continuous production.

This method of forming a siding is such that, after a recess and a projection are roll-formed on a metallic plate 1 to form a rugged face, a projected part for fitting is roll-formed on one side end and a recessed part for fitting on the other side end. In this case, in roll-forming the rugged face, both flat ends of the metallic plate 1 are held vertically between the holding rolls 5a, 5a made of an elastic rubber-like body.