To provide a different thickness wide metal plate having good quality characterized in that the cross-sectional shape thereof is highly flexibly designed and the cross-sectional shape is easily changed and the metal plate does not include weld zones or the like, and to provide a method of producing the different thickness wide metal plate.

A tubular base stock having a joined part extended in the longitudinal direction is produced by performing butt-joining of both ends of a flat plate made of metal which is bent into a tubular shape, a rotary mandrel mounted inside the tubular base stock pressurizes a working roll arranged outside the tubular base stock to the plate thickness direction of the tubular base stock while rotating the tubular base stock and also feeds the working roll in the longitudinal direction. the working roll and the mandrel perform thickness reducing work for reducing the plate thickness of the whole or a part in the longitudinal direction of the tubular base stock, and thus a tubular member having the plate thickness different in the longitudinal direction is produced. Further, a bent open member is produced by cutting and removing the joined part of the tubular member having different thickness, then the open member is expanded into a flat plate shape, thus the different thickness metal plate is produced.