A video image decoding device that decodes a code string to be decoded in units of blocks, comprising: a receiver that receives, as the code string to be decoded, a first code string to be decoded including information based on an encoded residual coefficient and header information associated with the information or a second code string to be decoded including a variable-length-encoded differential image obtained in an encoding process step of the code string to be decoded and header information associated with the residual image; a header analyzer that acquires for each block unit at least prediction information serving as information related to a predictive image used in generating the code string to be decoded from the header information; wherein the header information includes a mode information indicating whether the code string is to be decoded as a first code string in a first mode, or whether the code string is to be decoded as a second code string in a second mode; a coefficient code string analyzer that, in the first mode, variable-length-decodes the first code string to output a residual coefficient, and in the second mode, variable-length-decodes the second code string to output the differential image; a predictive residual decoder that, when the coefficient code string analyzer outputs the residual coefficient, performs inverse quantization and inverse orthogonal transformation to the outputted residual coefficient; a predictive decoder that generates a predictive image corresponding to the code string to be decoded based on the prediction information acquired by the header analyzer; and an adder that, in the first mode, adds the residual decoded image generated by the predictive residual decoder and the predictive image generated by the predictive decoder to each other to generate and output a reconstructed image and, in the second mode, adds the differential image output from the coefficient code string analyzer and the predictive image generated by the predictive decoder to each other to generate and output a reconstructed image, wherein a switching between the first mode and the second mode is performed only based on the mode information. a receiver that receives, as the code string to be decoded, a first code string to be decoded including information based on an encoded residual coefficient and header information associated with the information or a second code string to be decoded including a variable-length-encoded differential image obtained in an encoding process step of the code string to be decoded and header information associated with the residual image; a first code string to be decoded including information based on an encoded residual coefficient and header information associated with the information or a second code string to be decoded including a variable-length-encoded differential image obtained in an encoding process step of the code string to be decoded and header information associated with the residual image; a header analyzer that acquires for each block unit at least prediction information serving as information related to a predictive image used in generating the code string to be decoded from the header information; wherein the header information includes a mode information indicating whether the code string is to be decoded as a first code string in a first mode, or whether the code string is to be decoded as a second code string in a second mode; a coefficient code string analyzer that, in the first mode, variable-length-decodes the first code string to output a residual coefficient, and in the second mode, variable-length-decodes the second code string to output the differential image; a predictive residual decoder that, when the coefficient code string analyzer outputs the residual coefficient, performs inverse quantization and inverse orthogonal transformation to the outputted residual coefficient; a predictive decoder that generates a predictive image corresponding to the code string to be decoded based on the prediction information acquired by the header analyzer; and an adder that, in the first mode, adds the residual decoded image generated by the predictive residual decoder and the predictive image generated by the predictive decoder to each other to generate and output a reconstructed image and, in the second mode, adds the differential image output from the coefficient code string analyzer and the predictive image generated by the predictive decoder to each other to generate and output a reconstructed image, wherein a switching between the first mode and the second mode is performed only based on the mode information.