PURPOSE: To obtain a frozen food of bivalve wherein valves are opened by heat- treatment even in the case of frozen dead bivalves, by putting bivalves with valves and seasoning solution materials in a bag, dearating and sealing hermetically it so that preparation treatment for valve opening is carried out, followed by freezing them.

CONSTITUTION: The bivalves 2 with shells and the seasoning solution materials 3 are put in the airtight bag 1, preparation treatment for valve opening is carried out, and they are frozen to prepare a bagged frozen food of bivalve. The bagged frozen food of bivalve having undergone preparation treatment for valve opening, for example, is a food wherein the bivalves 2 with shells frozen individually and the frozen seasoning solution materials 3 are put in the airtight bag, the bag is dearated, sealed hermetically, and they are refrozen. The bagged frozen food of the bivalves 2 are heated together with the bag, the bivalves 2 are boiled and steamed in the bag 1, and seasoned while the valves are opended. In the heating, water is boiled, the frozed food tother with the bag immersed in it, the whole bag is sprayed with high-temperature steam, and or the bag is warmed by a heater.