Provided are a method for redundant slice coding by polyphase down-sampling (PD) and a codec using the method. An encoder for redundant slice coding by PD includes a PD coding unit rearranging a residual block that is a difference between a current block and a prediction block into four sections by performing PD on the residual block, a quantization transform unit transforming and quantizing at least one of the sections of the residual block, and a reference block generation unit generating a reference block based on a value obtained by performing inverse transform and inverse quantization, and then inverse PD on the at least one transformed and quantized section by the quantization transform unit.