Provided is a manufacturing method of an interior member of a plasma processing apparatus, which improves processing yield. The interior member is disposed inside a processing chamber of the plasma processing apparatus and includes, on a surface thereof, a film of a material having resistance to plasma. The manufacturing method includes: a step of moving a gun by a predetermined distance along the surface of the interior member to spray the material to form the film, and disposing a test piece having a surface having a shape simulating a surface shape of the interior member within a range of the distance within which the gun is moved and forming the film of the material on the surface of the test piece; and a step of adjusting, based on a result of detecting a crystal size of the film on the surface of the test piece and presence or absence of a residual stress or inclusion of a contaminant element, a condition of forming the film on the surface of the interior member by the gun.