A laser includes a laser medium, a light source for emitting light which pumps the laser medium, a pair of mirrors which are disposed on opposite sides of the laser medium and form a resonator, an etalon which is disposed in the resonator in order to make the oscillation mode of a laser beam a single longitudinal mode and a nonlinear optical crystal which is disposed in the resonator and converts the laser beam to a second harmonic. The effective thickness t of the etalon satisfies the condition that 2t(n.sub.SH -n.sub.FM)/.lambda..sub.SH is substantially an integer wherein .lambda..sub.SH represents the wavelength of the second harmonic, n.sub.FM represents the refractive index of the etalon for the fundamental wave and n.sub.SH represents the refractive index of the etalon for the second harmonic.