Optical device, and method for coating optical base

PROBLEM TO BE SOLVED: To solve a trouble caused by the stepwise refractive index characteristic generated along a coating by changing the refractive index steplessly from the refractive index of a support base surface to the refractive index of the coating surface. SOLUTION: A base 7 is held by an annular base support tool 5 provided in the center of an annular processing area 1, the base support tool 5 is slowly rotated within the annular processing area 1. The process atmosphere of the processing area 1 is supplied through a gas feeder 9 to generate RF-plasma discharge within the processing area 1, and a plasma polymerization coating is provided on the surface of the base 7. The composition of plasma atmosphere and/or the process parameter of plasma polymerizing process are controlled and changed, whereby the refractive index is steplessly changed from the refractive index of the base 7 to a desired final refractive index of the coating along the developing coating.