To provide a formation method of an insulating layer capable of forming the insulating layer of excellent characteristics in the case of use as a gate insulating layer for instance.

The method of forming the insulating layer on the surface of a substrate W to be processed comprises: a first nitriding process for nitriding silicon exposed to the surface of the substrate to be processed and forming a silicon nitride film on the surface of the silicon substrate, and a first annealing process of forming the silicon oxide nitride film 4 by annealing the substrate to be processed on which the silicon nitride film is formed in the state of maintaining a pressure within the range of 50 to 70 Torr (6,665 to 9,331 Pa) in an N2O atmosphere. Thus, the insulating layer of the excellent characteristics as the gate insulating layer for instance is formed.