An achromatic phase modulator (100; 200; 300; 400) that modulates phase of a linearly polarized light and outputs the linearly polarized light having undergone phase modulation, comprising: at least two liquid crystal elements (1, 2; 11, 12, 13) disposed in series on and perpendicularly to a light path of the linearly polarized light, which are constituted with liquid crystal materials with refractive index wavelength dependence characteristics thereof different from each other; a linear polarizer that is disposed on the light path and converts an incoming light propagating on said light path to the linearly polarized light and outputs the linearly polarized light to one of the two liquid crystal elements (1, 2; 11, 12, 13) so that the linearly polarized light enters substantially perpendicularly; and a control unit (15; 215) for applying drive electric signals individually to the liquid crystal elements so as to achieve achromatic phase modulation for the linearly polarized light, wherein in each liquid crystal element (1, 2; 11, 12, 13), an alignment direction of liquid crystal molecules therein can be altered between a first alignment direction substantially parallel to a direction in which the linearly polarized light advances, and a second alignment direction substantially parallel to the polarization direction of the linearly polarized light, in a plane including the first alignment direction and the second alignment direction, in correspondence to the drive electric signal applied to the liquid crystal element (1, 2; 11, 12, 13), and the polarization direction of the linearly polarized light entering the phase modulator coincides with the polarization direction of the linearly polarized light exiting the phase modulator. at least two liquid crystal elements (1, 2; 11, 12, 13) disposed in series on and perpendicularly to a light path of the linearly polarized light, which are constituted with liquid crystal materials with refractive index wavelength dependence characteristics thereof different from each other; a linear polarizer that is disposed on the light path and converts an incoming light propagating on said light path to the linearly polarized light and outputs the linearly polarized light to one of the two liquid crystal elements (1, 2; 11, 12, 13) so that the linearly polarized light enters substantially perpendicularly; and a control unit (15; 215) for applying drive electric signals individually to the liquid crystal elements so as to achieve achromatic phase modulation for the linearly polarized light, wherein in each liquid crystal element (1, 2; 11, 12, 13), an alignment direction of liquid crystal molecules therein can be altered between a first alignment direction substantially parallel to a direction in which the linearly polarized light advances, and a second alignment direction substantially parallel to the polarization direction of the linearly polarized light, in a plane including the first alignment direction and the second alignment direction, in correspondence to the drive electric signal applied to the liquid crystal element (1, 2; 11, 12, 13), and the polarization direction of the linearly polarized light entering the phase modulator coincides with the polarization direction of the linearly polarized light exiting the phase modulator.