It has been very difficult to accumulate strong laser in the conventional optical resonator, because firstly it has been very difficult to control a resonator length less than 1 Å in resonation position which is required for the laser amplification more than 1,000 times and secondly, the conventional method has utilized laser strength of amplified laser in the optical resonator as the resonance control signal.The present invention provides an optical resonator system to accumulate strong laser. In the system, unamplified modulation wave or harmonic which are derived from oscillation laser are selectively used to tune a resonator length of the optical resonator.