This paper presents an efficient bandwidth allocation method for the two-layer video coding of different spatial resolution. We first find a model of distortion-bitrate relationship for the MPEG-2 spatial scalable coding in a fixed total bitrate system. Then we propose an adaptive bitrate allocation method for a constant distortion ratio between two layers with the given total bandwidth. In the proposed method, approximated model parameters are used for simple implementation. The validity of the approximation is proven in terms of the convergence to the desired distortion ratio. It is shown by simulation that the proposed bitrate allocation method can keep almost a constant distortion ratio between two layers in comparison to a tired bitrate allocation method.