The amplitude squeezing of coherent light in a high-Q cavity by interaction with atoms entering regularly one by one is studied. The interaction is assumed to be of two-photon transition type, and the mean photon number N of the initial coherent light is assumed to be large. The interaction interval tau for an individual atom is taken as the favourable value pi /g, where g denotes the effective coupling constant. This value makes an incoming atom in the upper level change to the lower level at the exit. The Golubev-Sokolov equation for the density matrix of photons and generating function approach are utilized without any approximation. From this equation, the variance squared of photon number < Delta n(t)²>^av as a function of t is derived. Over a certain range of parameters, < Delta n(t)²>_av/<n(t)>^av may have a minimum value much lower than its steady-state value 1/2 at some moment. The correlation functions and power spectrums of the photon number noise and of the output photon current noise are also investigated.