Abstract: A semi-monolithic ring cavity for a second harmonic generation of laser frequency including a nonlinear crystal, an input mirror, an output mirror, a first point, at which a fundamental wave is incident onto the input mirror, a second point, at which the fundamental wave is incident onto the output mirror along with a second harmonic wave, and a third point, at which the fundamental wave reflects totally from a side surface of the nonlinear crystal. One of the input and output mirrors is coated with a dielectric and separated from the nonlinear crystal whereas the other mirror is formed on the nonlinear crystal. The fundamental wave forms a ring cavity in such a manner that the second harmonic wave is generated only along an axis extending through the first and second points. The semi-monolithic ring cavity exhibits a stable frequency characteristic and a high transform efficiency when a second-harmonic generation of continuous-wave laser frequency is carried out.