A process for efficiently preparing an unsaturated dicarboxylic acid diester, which comprises reacting a conjugated diene, carbon monoxide, an alcohol and oxygen in the presence of a catalyst comprising (a) a platinum metal, a salt thereof or a complex compound of a platinum metal, (b) at least one metal salt selected from the group consisting of a copper salt, an iron salt and a manganese salt and (C) at least one compound selected from the group consisting of (1) an alkali metal compound or an alkaline earth metal compound, (2) a rare earth metal compound, (3) a metal compound of the groups IIIA, IVA, IVB, VA, VB or VIA, and (4) a metal carbonyl complex. The unsaturated dicarboxylic acid diester is useful as an intermediate material in industrial chemical processes and can be efficiently prepared without the use of a dehydrating agent.