PURPOSE: To form a thin film which is uniform and has good film quality on a substrate of large area by plasma forming of a reactive gas and reacting it with target particles to form the thin film on the surface of the substrate to be treated.

CONSTITUTION: Reactive gas is introduced into a gas introducing part 5 and is made plasma. A vacuum vessel is provided with a gas discharge part 4. A gas introducing port 5a of the gas introducing part 5 is arranged apart a diffusion section on the upstream side of the gas discharge part 4 interposing a target 1. Reactive gas (gaseous nitrogen) introduced from the gas introducing port 5a is diffused together with an inert gas (gaseous argon) introduced from the gas introducing part 5 through the diffusion section, and is made uniform plasma on a plane intersecting the transfer direction of a substrate 3 to be treated. The plasma is reacted with target particles flying from the target 1 to form a thin on the surface of the substrate 3 to be treated. In this way, a reactive sputtering device by which a uniform thin film is formed on a substrate of large area is provided.