To provide a method for producing a substrate having a fine structure in which the shape is controlled and which has a high aspect ratio in relation to depth.

A method for forming a fine structure is characterized by forming a fine structure having a cross-sectional shape corresponding to a resist pattern and an aspect ratio of 1 or more by forming a resist pattern on the surface of anodically oxidized porous alumina and then applying a wet etching treatment to the resulting porous alumina in order to selectively solve and remove parts where the resist pattern is not formed. A mold for nanoimprint has a fine structure formed by the method. A method for producing a porous alumina composite is characterized by forming a resist pattern on the surface of anodically oxidized porous alumina, then forming through-holes having a cross-sectional shape corresponding to the resist pattern by applying a wet etching treatment to the resulting porous alumina in order to selectively solve and remove parts where the resist pattern is not formed, and filling a substance different from alumina into the through-holes.