The present invention provides a method capable of accurately producing a metal closed-section member in a relatively simple manner. The method of producing a metal closed-section member formed in a cross-sectionally closed shape, from a metal plate workpiece, comprises a first press-forming step of using a first forming die assembly to press-form the plate workpiece into a convex shape, and a second press-forming step of providing a second forming die assembly, and, after the first press-forming step, under a condition that the press-formed plate workpiece is positioned between a receiving die and a core die of the second forming die assembly, causing relative movement of the core die with respect to the receiving die to press-form the press-formed plate workpiece into a given shape so that two sidewalls of a convex-shaped portion of the press-formed plate workpiece formed on respective ones of both sides of a top wall of the convex-shaped portion of the press-formed plate workpiece during the first press-forming step are displaced toward an inward side of the press-formed plate workpiece and cross-sectionally closed together.