An oxidizing method for an object to be processed according to the present invention includes: an arranging step of arranging a plurality of objects to be processed in a processing container whose inside can be vacuumed, the processing container having a predetermined length, a main supplying unit of an oxidative gas and a supplying unit of a reducing gas being provided at one end of the processing container, a sub supplying unit of the oxidative gas being provided on a way in a longitudinal direction of the processing container; an atmosphere forming step of supplying the oxidative gas and the reducing gas into the processing container in order to form an atmosphere having active oxygen species and active hydroxyl species in the processing container; and an oxidizing step of oxidizing surfaces of the plurality of objects to be processed in the atmosphere. In the atmosphere forming step, the oxidative gas is adapted to be supplied from the main supplying unit of the oxidative gas and the sub supplying unit of the oxidative gas.