Disclosed are a method and apparatus for forming an insulating film on the surface of a semiconductor substrate capable of improving the quality and electrical properties of the insulating film with no employment of high-temperature heating and with good controllability. After the surface of a silicon substrate is cleaned, a silicon dioxide film having a thickness of 1-20 nm is formed on the substrate surface. The silicon substrate is exposed to plasma generated by electron impact, while the silicon substrate is maintained at a temperature of 0° C. to 700° C. Thus, nitrogen atoms are incorporated into the silicon dioxide film, obtaining a modified insulating film having good electrical properties.