PROBLEM TO BE SOLVED: To obtain the subject new compound useful as an intermediate for agrochemicals, etc., in high yield by irradiating a specific compound and an oxygen-contg. compound with light in the presence of an aryl ketone compound. SOLUTION: This new compound is shown by formula I [R and R are each a (substituted) 1-6C alkyl or (substituted) aryl; R and R are each H or a 1-6C alkyl; X is H, a 1-6C alkyl or OR (R is H or a 1-6C alkyl); Ar is a (substituted) aryl], e.g. 4-ethylenedioxymethyl-3-(p-tolylsulfinyl)-2-pentanone. This compound of formula I is obtained by irradiating an α-sulfinyl enone compound of formula II [e.g. 3-p-tolysulfinyl)-3-penten-2-one] and an oxygen- contg. compound of formula III (e.g. methanol) with light using e.g. a mercury vapor lamp in the presence of an aryl ketone compound (e.g. benzophenone).