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Constant bit-rate (CBR) encoding methods used for the real-time digital video applications such as terrestrial high definition television (HDTV) broadcasting often produce large picture quality fluctuation due to constraints on buffer size and buffering delay. In this paper, we propose a simple frame-layer CBR encoding method which reduces large picture quality fluctuation of an encoded video under constant bit-rate and limited buffer size. We first define an intermediate encoder buffer level to be lower than the half of the maximum encoder buffer level. Then, the encoder buffer level is kept from being close to the maximum (or minimum) buffer level during encoding by slightly decreasing (or increasing) the number of bits estimated to encode each frame if the resulting buffer level becomes higher (or lower) than the intermediate buffer level. The experimental results show that the proposed CBR encoding method reduces the average local PSNR variation by 20.6% compared with previous CBR encoding method on average under constant bit-rate and limited buffer size.