An optical device includes a phase modulation element having an optical waveguide part and electrodes, the optical waveguide part being configured such that a laser light beam emitted by a laser light source is inputted to the optical waveguide part and having an optical waveguide layer formed of an electro-optic material, the electrodes being provided on respective sides of the optical waveguide part to apply a voltage to the optical waveguide layer, the phase modulation element being configured to modulate a phase of the laser light beam by using a refraction index modulation region formed in the optical waveguide layer when a voltage is applied to the optical waveguide layer through the electrodes, and a pump light source configured to irradiate at least the refraction index modulation region of the optical waveguide layer with a pump light beam.