The present invention provides a device and method for producing, from two metal plates, a flangeless closed-cross-section-structure component having a deformed cross-sectional shape having a welded section and a bend in the direction of component length. Conventional techniques are not able to simultaneously proceed with bending and cross-section deformation. The present invention can simultaneously proceed with bending and cross-section deformation. Two metal sheets (1a, 1b) that are provided with a sheet-length-direction bend corresponding to a component (10) length-direction bend and that have both sheet width edges matched at a component welded section (11) are press-molded in a manner so that a crease (2) is imparted to a sheet section corresponding to a non-welded bent section (12) within the component cross section, then are overlapped vertically in a manner so that the protruding surface sides thereof are at the outside and the left edges and right edges of the width thereof are respectively welded together in the lengthwise direction thereof, forming the welded section (11), and while guiding and conveying the obtained closed-cross-section structure by means of a left-right pair of rotating rollers (40a, 40b) or furthermore by a lower rotating roller (40c), with the creases and the welded sections as starting points, the closed-cross-section structure is caused to become 3D and forms a component having a target shape by means of altering the inter-roller gap of the rotating rollers or by furthermore raising/lowering the rotating roller.