To provide a radical sterilization apparatus which can generate hydroxy (OH) radical and oxygen (O) radical at a high density to thereby surely and economically sterilize a subject to be treated. While maintaining a storage unit in which a medical instrument is stored at a low pressure by using a low pressure-maintaining unit, vapor is supplied form a vapor-supplying unit into the storage unit. Then, oxygen in this vapor is converted into radicals by a radical-forming unit to give hydroxy (OH) radical and oxygen (O) radical. Thus, the hydroxy radical and the oxygen (O) radical can be maintained each as a single radical over a long period of time and can be generated at a high density, which makes it possible to surely and economically sterilize the medical instrument.
