A process for producing an α,β-unsaturated aldehyde compound, comprising: subjecting two different aldehyde compounds to a cross condensation reaction in the presence of an amine and a protonic acid having 6 to 20 carbon atoms or a salt thereof; wherein the two different aldehyde compounds are aldehyde compounds I and II respectively represented by the formulae and: R 1 -CHO R 2 -CHO wherein R 1 and R 2 are groups different from each other; R 1 is a linear or branched alkyl group having 6 to 20 carbon atoms, a linear or branched alkenyl group having 6 to 20 carbon atoms or an alicyclic structure-containing group having 6 to 20 carbon atoms; and R 2 is a hydrogen atom, a methyl group or an ethyl group; or R 1 is an aromatic ring structure-containing group having 6 to 20 carbon atoms or a heterocyclic structure-containing group; and R 2 is a hydrogen atom, a methyl group or an ethyl group; and when the aldehyde compound II is HCHO, the resultant α,β-unsaturated aldehyde compound is a compound represented by the formula (III-a): wherein R 1a -C is a residual group of R 1 in the formula which is obtained by removing two hydrogen atoms bonded to a carbon atom being located at the α position of R 1 therefrom; and when the aldehyde compound II is an aldehyde compound other than HCHO, the resultant α,β-unsaturated aldehyde compound is a compound represented by the formula (III-b): wherein R 2a -C is a residual group of R 2 in the formula which is obtained by removing two hydrogen atoms bonded to a carbon atom being located at the α position of R 2 therefrom, and R 1 is the same as defined above. subjecting two different aldehyde compounds to a cross condensation reaction in the presence of an amine and a protonic acid having 6 to 20 carbon atoms or a salt thereof; wherein the two different aldehyde compounds are aldehyde compounds I and II respectively represented by the formulae and: R 1 -CHO R 2 -CHO wherein R 1 and R 2 are groups different from each other; R 1 is a linear or branched alkyl group having 6 to 20 carbon atoms, a linear or branched alkenyl group having 6 to 20 carbon atoms or an alicyclic structure-containing group having 6 to 20 carbon atoms; and R 2 is a hydrogen atom, a methyl group or an ethyl group; or R 1 is an aromatic ring structure-containing group having 6 to 20 carbon atoms or a heterocyclic structure-containing group; and R 2 is a hydrogen atom, a methyl group or an ethyl group; and when the aldehyde compound II is HCHO, the resultant α,β-unsaturated aldehyde compound is a compound represented by the formula (III-a): wherein R 1a -C is a residual group of R 1 in the formula which is obtained by removing two hydrogen atoms bonded to a carbon atom being located at the α position of R 1 therefrom; and when the aldehyde compound II is an aldehyde compound other than HCHO, the resultant α,β-unsaturated aldehyde compound is a compound represented by the formula (III-b): wherein R 2a -C is a residual group of R 2 in the formula which is obtained by removing two hydrogen atoms bonded to a carbon atom being located at the α position of R 2 therefrom, and R 1 is the same as defined above. wherein R 1 and R 2 are groups different from each other; R 1 is a linear or branched alkyl group having 6 to 20 carbon atoms, a linear or branched alkenyl group having 6 to 20 carbon atoms or an alicyclic structure-containing group having 6 to 20 carbon atoms; and R 2 is a hydrogen atom, a methyl group or an ethyl group; or R 1 is an aromatic ring structure-containing group having 6 to 20 carbon atoms or a heterocyclic structure-containing group; and R 2 is a hydrogen atom, a methyl group or an ethyl group; and when the aldehyde compound II is HCHO, the resultant α,β-unsaturated aldehyde compound is a compound represented by the formula (III-a): wherein R 1a -C is a residual group of R 1 in the formula which is obtained by removing two hydrogen atoms bonded to a carbon atom being located at the α position of R 1 therefrom; and when the aldehyde compound II is an aldehyde compound other than HCHO, the resultant α,β-unsaturated aldehyde compound is a compound represented by the formula (III-b): wherein R 2a -C is a residual group of R 2 in the formula which is obtained by removing two hydrogen atoms bonded to a carbon atom being located at the α position of R 2 therefrom, and R 1 is the same as defined above. wherein R 1a -C is a residual group of R 1 in the formula which is obtained by removing two hydrogen atoms bonded to a carbon atom being located at the α position of R 1 therefrom; and when the aldehyde compound II is an aldehyde compound other than HCHO, the resultant α,β-unsaturated aldehyde compound is a compound represented by the formula (III-b): wherein R 2a -C is a residual group of R 2 in the formula which is obtained by removing two hydrogen atoms bonded to a carbon atom being located at the α position of R 2 therefrom, and R 1 is the same as defined above.