In an atmosphere of processing gas, on a wafer W consisting mainly of silicon, through a planar-array antenna RLSA 60 having a plurality of slits, microwaves are irradiated to generate plasma containing oxygen, or nitrogen, or oxygen and nitrogen and to implement therewith on the surface of the wafer W direct oxidizing, nitriding, or oxy-nitriding to deposit an insulator film 2 of a thickness of 1 nm or less in terms of oxide film. A manufacturing method and apparatus of semiconductors that can successfully regulate film quality of the interface between a silicon substrate and a SiN film and can form SiN film of high quality in a short time can be obtained.