An object of the present invention is to easily eliminate fumes inside a chamber, so as to improve a positional accuracy of irradiation with a light beam and a machining accuracy in a method for manufacturing a three-dimensional shaped object. A stacked-layers forming device 1 includes a powder layer forming unit 3, a light beam irradiating unit 4, a base 22 which is fixed and on which a powder layer 32 is formed, a lifting/lowering frame 34 which surrounds the circumference of the base 22 and is freely capable of being lifted and lowered, a cover frame 36 which has a window 36a allowing transmission of light beam in its top surface, and whose bottom surface is opened, and which is disposed on the lifting/lowering frame 34 to form a chamber C, and a gas tank 71 for supplying an ambient gas. The lifting/lowering frame 34 is lowered to reduce the volume of the chamber C, so as to discharge fumes generated inside the cover frame 36, which performs replacement with the ambient gas. Since the volume of the chamber C is reduced, it is possible to easily eliminate the fumes, which makes it possible to improve the positional accuracy of irradiation with the light beam L, and the machining accuracy.