A first laser light source for emitting an optical beam of a first wavelength, a second laser light source for emitting an optical beam of a second wavelength different from the first wavelength and first and second polarizing devices through which the optical beams of first and second wavelengths transmit are provided. The first polarizing device changes the phase of the optical beam of first wavelength by about (M+½) times the first wavelength (M being integer) and the second polarizing device changes the phase of the optical beam of second wavelength by about (N+½) times the second wavelength (N being integer).