A computer-readable medium having stored thereon an image decoding program which, when executed by a computer, performs image decoding, wherein synthesizing a prediction image is performable using a positive rounding method and a negative rounding method for interpolating intensity values of pixels; wherein the interpolation of intensity values of pixels is performed using a rounding method specified by the rounding method information; wherein the rounding method information is included in coded information of the currently decoded image; wherein the rounding method information specifies one of two values; wherein one of the two values specifies a positive rounding method, and another one of the two values specifies a negative rounding method; and wherein the rounding method information consists of one bit.