A pulse laser beam is emitted in a desired wavelength sequence using a laser source unit. A Q switch and a birefringent filter are inserted into an optical resonator including a pair of mirrors and facing each other with a laser rod interposed therebetween. The birefringent filter changes an oscillation wavelength of the optical resonator in association with rotational displacement. A trigger control circuit rotates the birefringent filter at a predetermined rotation speed depending on the number of wavelengths included in the wavelength sequence of the pulse laser beam to be emitted. In addition, the trigger control circuit irradiates the laser rod with excitation light, and turns on the Q switch at a timing when a rotational displacement position of the birefringent filter is set to a position corresponding to the wavelength of the pulse laser beam to be emitted, to cause the pulse laser beam to be emitted.