Radiation field distributions of all mirror panels composing a main reflector are measured and held in advance, radiation field distributions of a reflector antenna having the main reflector are measured by transmitting a radio wave from a collimation antenna arranged at a prescribed distance from the reflector antenna to the reflector antenna while changing an attitude of the reflector antenna, complex excitation coefficients of each mirror panel are calculated from the radiation field distributions of the mirror panels, the radiation field distributions of the reflector antenna and the attitudes of the reflector antenna, and mirror surface accuracy of the main reflector is calculated from the complex excitation coefficients of the mirror panels.