NOVELTY - A prepolymer is obtained by reacting monomer and/or raw material polymer under specified condition, such that formation of infinite network is prevented. Raw material monomer and raw material polymer respectively contain two or more reactive terminal groups (A) and/or reactive terminal groups (B) in a molecule, or raw material monomer and/or raw material polymer contain 3 or more reactive terminal groups (A) and/or reactive terminal groups (B) in a molecule. The prepolymer has average particle size of 1 nm to 10 mu m.    USE - Prepolymer is used for catalyst layer (all claimed). Uses include but are not limited to fuel cell, electrolysis apparatus, hydrohalic acid electrolyzer, brine-electrolysis apparatus and electrolytic membrane.    ADVANTAGE - The prepolymer has excellent electrical conductivity and solvent solubility.    DETAILED DESCRIPTION - A prepolymer is obtained by reacting monomer and/or raw material polymer under specified condition, such that formation of infinite network is prevented. The raw material monomer and raw material polymer respectively contain two or more reactive terminal groups (A) and/or reactive terminal groups (B) in a molecule, or at least one of raw material monomer and raw material polymer contain 3 or more reactive terminal groups (A) and/or reactive terminal groups (B) in a molecule, or raw material monomer and raw material polymer contains at least two reactive terminal groups (A) or at least two reactive terminal groups (B). When reactive terminal groups (A) and reactive terminal groups (B) are reacted, bissulfonimide group, sulfone carbonimide group or bis carbonimide group is formed. INDEPENDENT CLAIMS are included for the following:    (1) prepolymer solution, which is obtained by dissolving or dispersing prepolymer in a solvent; and    (2) catalyst layer, which is obtained by coating a catalyst ink to he surface of support film. The catalyst ink is obtained by dispersing a catalyst supported by a support to the prepolymer solution.