Described herein is a technique capable of improving electrical characteristics of a polysilicon film while suppressing damage to an underlying silicon oxide film. According to the technique described herein, there is provided a there is provided a method of manufacturing a semiconductor device, including: (a) preparing a substrate including a silicon oxide film and a polysilicon film formed on the silicon oxide film, wherein the polysilicon film includes a contact surface contacting the silicon oxide film and an exposed surface facing the contact surface; and (b) supplying a reactive species generated by plasma excitation of a gas containing hydrogen and oxygen to the exposed surface of the polysilicon film.