To obtain a styrene oxide derivative on an industrial scale at a low cost without using a catalyst by reacting a styrene derivative with hydrogen peroxide and phthalic anhydride at the same time.

The objective derivative of formula II is produced by reacting a styrene derivative of formula I (R1 to R4 are each H or an organic residue; (n) is 0-5) with 1.2-2.5 equivalent each of hydrogen peroxide and phthalic anhydride at 20-50°C in the presence of a solvent such as chloroform. An example of the derivative is N-methyl-N-[3-(1,1,1-trichloro-3,4-epoxybutan-3-yl) benzyl]acetamide.