Field observation have been carried out to investigate spectral characteristics of natural wind velocity. Laboratory experiments on response of wind waves under periodically fluctuating winds have also been done. Spectral density of natural fluctuating wind velocity indicates a f^-1 decay low. The measurements of the wind velocity and air pressure above wind waves show that the fluctuating components of wind velocity change in phase to the water surface elevations, while those of the air pressure change almost out of phase to wind waves. In the experiments, long-period waves appear to be generated by the periodically fluctuating components of wind velocity, not by those of air pressure through the spectrum analysis of the fluctuating components of the water surface elevation, wind velocity and air pressure.