Appropriate rate control plays a very important role in encoding motion pictures under the constant bit-rate. One of the requirements for rate control is minimizing temporal fluctuations in picture quality, which may cause flicker artifacts. To satisfy the requirement, some methods for MPEG1 and MPEG-2 have been proposed. However, these methods cannot be applied to the H.264 encoding algorithm based on R-D optimization since they require the DCT coefficients and the accurate output bit amount for each quantization parameter of a current picture in advance. To overcome the problem, we propose a rate control method to satisfy the requirement in the H.264 encoding algorithm by introducing approximations to estimate the bit amount and the distortion.