1. A decoding device for decoding a coded data of a digital image constituted by a plurality of blocks, comprising: a selecting circuit configured to select one or more adjacent blocks for a subject block, which is a subject for the decoding; a deriving circuit for deriving, on the basis of a prediction mode(s) of the one or more adjacent blocks, an estimated prediction mode(s) to be estimated as an estimated value in intra-prediction mode estimation; a prediction mode decoding circuit for decoding, from the coded data, a prediction mode to be used in generation of a predicted image of the subject block, the prediction mode decoding circuit performing the decoding based on the estimated prediction mode(s) derived by the deriving circuit; and a predicted image generating circuit for generating a predicted image of the subject block, based on the prediction mode, the coded data comprising respective pieces of information regarding each of the plurality of blocks, the respective pieces of information being individually: a) (i) information indicating that an estimated prediction mode included in the group consisting of one or more estimated prediction modes for the block is identical with the prediction mode for the block, and (ii) information designating that estimated prediction mode; or b) (i) information indicating that no estimated prediction mode included in the group consisting of one or more estimated prediction modes for the block is identical with the prediction mode for the block, and (ii) information regarding the prediction mode.