In a process of resolving the two-dimensional orthogonal transformation into the one-dimension orthogonal transformations in a horizontal direction and a vertical direction, with respect to each signal vector within the block memory to be orthogonal-transformed in a vertical direction orthogonal transforming unit after having been orthogonal-transformed in a horizontal direction orthogonal transforming unit, a power calculating unit calculates the power of each signal vector, a threshold obtaining unit requires a threshold enabling the quantized coefficient signal to take a value other than zero, and a value comparing unit compares the both powers. When the power calculated by the power calculating unit is below the threshold, the vertical-directional orthogonal transformation and quantization processing as for the corresponding signal vector is omitted and a zero coefficient vector generating unit generates the quantized coefficient signal vector simply consisting of zero and supplies it as the calculation result.