A solid state laser medium is composed of Nd:YAG, the wavelength of a second harmonic wave is 557 to 559 nm and a free spectral range of a solid etalon falls in any of the values of a range A of 1.04 to 1.07 nm, a range B of 1.42 to 1.61 nm, a range C of 2.07 to 2.14 nm and a range D of 2.37 to 3.21 nm.