The present invention relates to a phase modulating apparatus capable of highly accurately and easily correcting the phase modulation characteristic of a reflective electric address spatial light modulator even when a condition of input light is changed. In the LCOS phase modulating apparatus, an input unit inputs the condition of the input light, and a processing unit sets an input value for each pixel. A correction value deriving unit determines a correction condition according to the condition of the input light. A control input value converting unit converts the input value set for each pixel into a corrected input value based on the correction condition. An LUT processing unit converts the corrected input value into a voltage value, and drives each pixel by using a drive voltage equivalent to the converted voltage value.