A method of producing a metal closed-section member formed in a cross-sectionally closed shape, from a metal plate workpiece, comprising: a first press-forming step of providing a first forming die assembly, and causing relative movement of a punch of the first forming die assembly with respect to the die to allow the punch to be moved into a forming space of the die, so that the plate workpiece is press-formed into a shape convexly protruding in a direction of the movement of the punch relative to the die and a convex wall portion (W2a) is formed in the top wall of the press-formed plate workpiece to protrude in a direction opposite to the convexly protruding direction of the press-formed plate workpiece; 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, wherein the core die is disposed on the side opposite to the convexly protruding direction of the press-formed plate workpiece characterized in that said first press-forming step is carried out under a condition that the plate workpiece is clamped between a die and a blank holder of the first forming die assembly, in said second press-forming step, the press-formed plate workpiece is held on the receiving die of the second forming die assembly with the convex wall portion (W2a) of the press-formed plate workpiece placed on a concave portion or a flat portion of the receiving die causing relative movement of the core die with respect to the receiving die, and the convex wall portion (W2a) of the press-formed plate workpiece is pressed in the convexly protruding direction of the press-formed plate workpiece to allow the convex wall portion (W2a) of the press-formed plate workpiece to be flattened and formed as a press-formed wall (W2d), by an interaction between a core die of the second forming die assembly and the concave portion or the flat portion, so that two sidewalls (W3, W4) of a convex-shaped portion of the press-formed plate workpiece formed on respective ones of both sides of the top wall during the first press-forming step are displaced toward an inward side of the press-formed plate workpiece and cross-sectionally closed together; and in a final press-forming step of, after the second press-forming step, respective ends (W3e, W4e) of the cross-sectionally-closed sidewalls (W3, W4) of the press-formed plate workpiece are pressed against a surface of the core die on an opposite side of a surface thereof used to press the top wall. a first press-forming step of providing a first forming die assembly, and causing relative movement of a punch of the first forming die assembly with respect to the die to allow the punch to be moved into a forming space of the die, so that the plate workpiece is press-formed into a shape convexly protruding in a direction of the movement of the punch relative to the die and a convex wall portion (W2a) is formed in the top wall of the press-formed plate workpiece to protrude in a direction opposite to the convexly protruding direction of the press-formed plate workpiece; 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, wherein the core die is disposed on the side opposite to the convexly protruding direction of the press-formed plate workpiece characterized in that said first press-forming step is carried out under a condition that the plate workpiece is clamped between a die and a blank holder of the first forming die assembly, in said second press-forming step, the press-formed plate workpiece is held on the receiving die of the second forming die assembly with the convex wall portion (W2a) of the press-formed plate workpiece placed on a concave portion or a flat portion of the receiving die causing relative movement of the core die with respect to the receiving die, and the convex wall portion (W2a) of the press-formed plate workpiece is pressed in the convexly protruding direction of the press-formed plate workpiece to allow the convex wall portion (W2a) of the press-formed plate workpiece to be flattened and formed as a press-formed wall (W2d), by an interaction between a core die of the second forming die assembly and the concave portion or the flat portion, so that two sidewalls (W3, W4) of a convex-shaped portion of the press-formed plate workpiece formed on respective ones of both sides of the top wall during the first press-forming step are displaced toward an inward side of the press-formed plate workpiece and cross-sectionally closed together; and in a final press-forming step of, after the second press-forming step, respective ends (W3e, W4e) of the cross-sectionally-closed sidewalls (W3, W4) of the press-formed plate workpiece are pressed against a surface of the core die on an opposite side of a surface thereof used to press the top wall.