To easily obtain the subject compounds useful as an intermediate for producing physiologically active substances and the like in a good yield by reacting a specific imine compound with a carbonyl compound in the presence of a specific catalyst.

(A) An imine compound of formula I (R1 to R3 are each a non-reactive atom or organic group) (for example, N-butylidene-s-butylamine) is reacted with (B) a carbonyl compound of the formula: R4CH2COR5 (R4 and R5 are each a non-reactive atom or organic group) (for example, propionaldehyde) in the presence of (C) a catalyst comprising a compound of the group III element in the periodic table) [for example, scandium (III) trifluoromethanesulfonate] usually in an organic solvent such as terahydrofuran preferably at a temp. about 10 to 50°C to obtain an α,β-unsaturated imine compound of formula II or III [for example, N-(2-methyl-2-hexylidene)-s-butylamine].