PURPOSE: To readily regulate quality of a fermented dairy product without impairing the original components, flavor, etc., thereof and using an additive, by removing malodorous substances, e.g. lactic acid, etc., contained in the product utilizing osmotic pressure.

CONSTITUTION: A fermented dairy product, e.g. yogurt, lactic acid beverage, etc., is brought into contact with a high-osmotic pressure substance, e.g. aqueous solution of sucrose, etc., through a semipermeable membrane, e.g. cellophane, etc., by a means for respectively containing the fermented dairy product and the high-osmotic pressure substance in two chambers divided by the semipermeable membrane as a partition wall or filling the high-osmotic pressure substance in a container and dipping the fermented dairy product wrapped in the semipermeable membrane or forming a dehydrating sheet from the semipermeable membrane and high-osmotic pressure substance and filling the fermented dairy product in a container prepared from the above-mentioned sheet, etc., to absorb malodorous substances contained in the fermented dairy product in the high- osmotic pressure substance.