[Problem] To provide a flattening film forming composition that is for a hard disk and that is able to prevent a magnetic material such as a cobalt component from diffusing to a filler section (nonmagnetic layer). [Solution] The flattening film forming composition for a hard disk is characterized by containing at least one polymer selected from the group consisting of a homopolymer, which has a divinyl aromatic compound-derived unit structure, and a copolymer having said unit structure, or a photopolymerizable coating material containing a mixture of the polymer and a photopolymerizable compound, the coating material containing 1-90 mol% of vinyl groups per mole of benzene rings. The photopolymerizable compound is a compound having an acrylate group, a methacrylate group, or a vinyl group. The polymer is a copolymer that further contains an addition polymerizable compound as a copolymerization component. A method for producing a hard disk includes: a first step for forming bumps and depressions on a magnetic body; a second step for coating the bumps and depressions with a flattening film forming composition; and a third step for flattening by means of etching and exposing the magnetic body surface.