First and second laser beams are incident on a difference frequency mixing unit, and the two laser beams overlap in the difference frequency mixing unit, thereby generating a terahertz wave whose frequency is a frequency difference between the two laser beams. Each laser beam has a spatial spread in a frequency gradient direction, and a spatial frequency distribution of each laser beam is a distribution in which a magnitude of a frequency gradually increases in the frequency gradient direction. A frequency difference between the two laser beams is equal in each position in an overlapping region of the two laser beams. A terahertz wave of a single frequency is generated from this overlapping region. By spatially shifting the first laser beam and the second laser beam with respect to each other in the frequency gradient direction, the frequency difference in the overlapping region is changed, which changes the single frequency.