When the signal format of a color image is YUV4:2:2, and an intra prediction parameter, which is used for an intra-frame prediction process on the chrominance signals in a prediction block, shows the same prediction mode as an intra prediction mode for the luminance signal in the prediction block, an intra prediction unit 4 converts an index indicating the intra prediction mode for the luminance signal, and performs the intra-frame prediction process on the chrominance signals in an intra prediction mode indicated by the index after conversion.