Computer-readable medium with image decoding program which performs: storing a reference image which is a previously decoded image; receiving coded information including motion vector and rounding method information specifying a rounding method used in synthesizing a prediction image of a currently decoded image; and synthesizing the prediction image via motion compensation using the motion vector information and reference image; wherein synthesizing a prediction image uses a positive and negative rounding method for interpolating intensity values of pixels; wherein interpolation of intensity values of pixels uses a rounding method specified by the rounding method information; wherein interpolation is performed using the rounding method specified by rounding method information included in the encoded bitstream of the current frame when the current frame is a P frame; and wherein the rounding method information is not received from the encoded bitstream of the current frame when the current frame is an I frame.