PURPOSE: To obtain a method which forms a gate insulation film excellent in insulation withstand voltage.

CONSTITUTION: After a wafer 4 is cleaned by RCA or HF, the wafer 4 is installed in a quartz tube 1 of a quick heating device. This quartz tube provides a halogen lamp 2 and it is installed in a high pressure vessel. During the formation of an insulation film, an N2O gas and H2 gas, which are under control, are introduced into the quartz tube 1. The conditions at that time require that a flow rate of the N2O gas is 9l/min and a flow rate of the H2 gas is 1l/min while the reaction temperature is 950°C. The oxide nitride film formed on the wafer 4 provides a 90 angstrom thick film and further produces an excellent state in the interface between the wafer and the film, which is flattened and uneven.