To suppress useless consumption of a hydrogen donor by supplying a decomposition-inhibitor of one type or two and more types selected from a group of acetic acid, formic acid and their salts together with the hydrogen donor.

A decomposition-inhibitor of about 100-1,000 mg/L of an aqueous solution of one type or two and more types selected from a group of acetic acid, formic acid and their salts is supplied into soil and ground water together with a hydrogen donor using a seccharide, organic salts, alcohols, proteins or their mixtures, and ethanol, propionic acid or alkali-metal salts of propionic acid, such as sodium propionate and potassium propionate. As a result it makes possible to improve the efficiency of utilization of the hydrogen donor by suppression of useless consumption of the hydrogen donor, and it makes possible to decompose organic chlorine compounds efficiently with a smaller amount of the hydrogen donor.