PURPOSE: To improve the preservability of various foods, by superimposing the output of a high frequency AC high voltage having variable high frequencies on a DC high voltage as a standard voltage and impressing the superimposed voltages on the various foods.

CONSTITUTION: Arbitrary voltages having arbitrary frequencies outputted from an AC output circuit having a power amplifier PA and a function generator FG capable of arbitrarily varying wave shapes and frequencies are superimposed on the voltage of a DC high voltage output circuit Pd and the superimposed voltages Ph are impressed on various foods (e.g., gains, fresh vegetables, fruits, fishes, shellfish and various processed foods thereof).