To provide a method for reducing microorganisms attached to food materials, such as grains and beans, vegetables, fish and shellfish, without impairing the flavor and taste.

An aqueous solution of calcined calcium having a pH of 12.0 or higher is mixed with ozone gas or oxygen gas, to produce water having a positively increased OH radical with germicidal effects. By treating food materials, such as grains and beans, vegetables, fish and shellfish to running water, microorganisms attached to the food materials are reduced without impairing the flavor and taste.