The present invention provides: hydroxyapatite, a principal component of teeth, which has a high degree of hardness, is to be used in a device for forming a membrane on a tooth surface by spraying the tooth surface with a powder, and is favorable for forming a membrane having an extremely low solubility in acid in a short period of time and favorable for forming a membrane from a powder that conforms to the color tone of the tooth in a short period of time； and a membrane-forming powder that uses hydroxyapatite having a color tone modifier added thereto. The present invention produces: a hydroxyapatite powder sintered at 600-1,350°C in an inert gas atmosphere； a powder obtained by subjecting the hydroxyapatite powder sintered at 600-1,350°C to plasma exposure, or to plasma exposure and the application of mechanical energy； and a membrane-forming powder obtained by adding a color tone modifier to these hydroxyapatite powders.
