Provided is a method for preparing (E3,Z5)-3,5-alkadienyl acetate and (E3,Z5)-3,5-dodecadienyl acetate which is a sex pheromone of Brazilian apple leafminer. Specifically, provided is a method for preparing (E3,Z5)-3,5-alkadienyl acetate, comprising steps of hydrolyzing 5,5-diethoxy-(Z3)-3-pentenyl methoxymethyl ether in the presence of an acid to obtain 4-formyl-(E3)-butenyl methoxymethyl ether; reacting the 4-formyl-(E3)-butenyl methoxymethyl ether with alkylidene triphenylphosphorane in accordance with the Wittig reaction to obtain (E3,Z5)-3,5-alkadienyl methoxymethyl ether; and obtaining (E3,Z5)-3,5-alkadienyl acetate using the (E3,Z5)-3,5-alkadienyl methoxymethyl ether as a starting substance.