To industrially, efficiently and safely obtain the subject compound useful as an intermediate of medicines and agrochemicals by continuously and simultaneously feeding respective raw materials into a reactor to carry out reaction without storing of hydrogen sulfide as one of the raw materials.

This compound of formula III is obtained by reaction between (A) a compound of formula I (R1 is H or methyl), (B) a compound of formula II (R2 and R3 are each a 1-3C alkyl) and (C) hydrogen sulfide by simultaneously and continuously feeding the respective raw materials into a reactor. Preferably the reaction is performed at 0-100°C, and the molar ratios of B/A and C/A are ≥1.0 respectively, and the component C is blowingly fed into the reaction liquor.