An object of the present invention is to provide a method for producing a block copolymer solid in which microphase-separated structures are formed with good productivity. The present invention provides a method for producing a block copolymer solid in which a microphase-separated structure is formed, or an alloy, blend or composite solid containing a block copolymer in which a microphase-separated structure is formed, which comprises the following steps (1) to (3): (1) immersing a block copolymer solid or an alloy, blend or composite solid containing a block copolymer in a solvent in which each component is insoluble; (2) putting the solid obtained in (1) above in a pressure-resistant container; and (3) irradiating the solid or the solvent in (2) above with microwave.