1. A picture coding device which codes a picture signal by use of an intra prediction mode selected from a plurality of intra prediction modes in units of block and codes information for specifying the selected intra prediction mode, the picture coding device comprising: an intra prediction mode selecting unit configured to select an intra prediction mode of a block to be coded; an intra prediction mode storing unit configured to store intra prediction modes of coded blocks; a prediction direction difference deriving unit configured to derive intra prediction modes of a plurality of reference blocks used for an intra prediction processing of the block to be coded from the intra prediction mode storing unit, and derive a prediction direction difference indicating a degree of difference in prediction directions of the derived intra prediction modes, and when the prediction direction difference is larger than a predetermined upper limit, replace a value of the prediction direction difference with the upper limit; a coding tree creating unit configured to determine most probable prediction modes as intra prediction mode candidates of the block to be coded based on the intra prediction modes of the reference blocks and the prediction direction difference, and create a coding tree assigned with a bitstream having a shorter code length than other prediction modes for the determined most probable prediction modes; and a coding unit configured to code information for specifying an intra prediction mode of the block to be coded according to the coding tree, wherein: when the intra prediction modes of the reference blocks are not the same, the coding tree creating unit specifies intra prediction mode of left neighboring block to the block to be coded as first most probable prediction mode, and intra prediction mode of upper neighboring block to be coded as second most probable prediction mode; when the intra prediction modes of the reference blocks are the same and are not an average value mode, the coding tree creating unit specifies the same intra prediction mode as first most probable mode and specifies two intra prediction modes closest to the same intra prediction mode as second and third most probable prediction modes; and when the intra prediction modes of the reference blocks are the same and are the average value mode, the coding tree creating unit specifies the average value mode and a vertical direction prediction mode as most probable prediction modes.