In a state where food ingredients, degradative enzymes, and liquid seasonings are housed and sealed in a hard vessel, it has become possible to develop food products packaged in the hard vessel for normal temperature distribution or chilled distribution by a method of carrying out all of the steps inside the hard vessel, which method comprises carrying out an enzyme impregnation step and an enzymatic reaction step by heating and carrying out an enzyme deactivation step and sterilization step by continuous heating； or by a method of carrying out, after introducing the degradative enzyme into the food ingredient, some steps of enzymatic reaction step, enzyme deactivation step, and sterilization step inside the hard vessel.
