A manufacturing method for a molded article, comprising: a folding step for forming a core layer by folding a sheet material molded of a single plastic sheet into a predetermined shape so that the core layer is sectioned into a plurality of cells that are formed by the polygonal-cylindrical or circular-cylindrical sidewalls and are disposed adjoiningly inside, wherein the core layer is provided with middle walls having a multilayer structure formed of the sidewalls of adjacent cells in which sidewalls are layered; joining step for forming a structure by joining skin layers (3, 4) to upper and lower surfaces of the core layer; and processing step for forming a bent portion by applying a bending process to the structure, the manufacturing method being characterized in that the middle walls have a non-adhesive two-layer structure such that the layers of the middle walls merely make contact with each other and can be separated from each other when the internal pressure of cells is increased, wherein the entire area between the layers of the middle walls 23 is a non-adhesive portion, and the processing step includes heat treating the structure to make the sheet material in a softened state, and increasing internal pressure of the cells by the heat treating whereby the layers of the middle walls separate throughout said area to form a communication portion between the layers of the middle walls, wherein the communication portion connects adjacent cells. a folding step for forming a core layer by folding a sheet material molded of a single plastic sheet into a predetermined shape so that the core layer is sectioned into a plurality of cells that are formed by the polygonal-cylindrical or circular-cylindrical sidewalls and are disposed adjoiningly inside, wherein the core layer is provided with middle walls having a multilayer structure formed of the sidewalls of adjacent cells in which sidewalls are layered; joining step for forming a structure by joining skin layers (3, 4) to upper and lower surfaces of the core layer; and processing step for forming a bent portion by applying a bending process to the structure, the manufacturing method being characterized in that the middle walls have a non-adhesive two-layer structure such that the layers of the middle walls merely make contact with each other and can be separated from each other when the internal pressure of cells is increased, wherein the entire area between the layers of the middle walls 23 is a non-adhesive portion, and the processing step includes heat treating the structure to make the sheet material in a softened state, and increasing internal pressure of the cells by the heat treating whereby the layers of the middle walls separate throughout said area to form a communication portion between the layers of the middle walls, wherein the communication portion connects adjacent cells. heat treating the structure to make the sheet material in a softened state, and increasing internal pressure of the cells by the heat treating whereby the layers of the middle walls separate throughout said area to form a communication portion between the layers of the middle walls, wherein the communication portion connects adjacent cells.