All clocks based on periodic phenomena in nature are composed of three elements: oscillator, reference, and counter. In the case of optical clocks, an ultra-stable laser is used as the local oscillator. (The reference is a narrow-linewidth transition in atoms, and the counter is an optical frequency comb.) The ultra-stable laser with a narrow linewidth and high frequency stability is essential to the operation of optical clocks, and the heart of a stable laser system is a high finesse optical cavity which must be situated in a good environment to isolate it from perturbations and should require an excellent design to be insensitive to perturbations. At the National Metrology Institute of Japan, AIST, an ultra-stable laser has been developed to drive a narrow linewidth clock transition ¹S₀-³P₀ in Yb atoms. Using very cold Yb atoms bound in optical lattices and optical frequency combs, we have been able to measure the frequency of the clock transition very precisely. The determined frequency was adopted as one of 'recommended standard frequencies' by the International Committee for Weights and Measures (CIPM).