To provide a thin film forming method and apparatus that can automatically form thin films having constant optical properties with high reproducibility Antireflection films are deposited on lenses 2a that are held by a coating dome 2 by vaporizing an evaporation material 4 by using an electron gun 3 The power to be applied to the electron gun 3 is controlled so that a transmission or reflection light quantity value that is measured at each time point by an optical film thickness meter 10 becomes equal or approximately equal to a standard light quantity value stored in a standard light quantity value data storing means