1. A decoding device for decoding coded image data based on an estimation of a prediction mode of a target region to be intra-frame predicted, which estimation is made based on one or more prediction modes of one or more neighbor regions which are neighboring to the target region, the decoding device comprising: a transform circuit configured to transform the one or more prediction modes of the one or more neighbor regions into one or more first prediction modes which are included in a set of prediction modes, wherein one or more prediction modes which are different from the one or more prediction modes of the one or more neighbor regions are included in the one or more first prediction modes, wherein the set of prediction modes is comprised of at least a first set of prediction modes and a second set of prediction modes, the first set of prediction modes being applicable to a first neighboring region of the target region to be intra-frame predicted and the second set of prediction modes being applicable to a second neighboring region of the target region to be intra-frame predicted; and an estimated prediction mode deriving circuit configured to derive an estimated prediction mode by estimating the prediction mode of the target region based on the one or more first prediction modes; wherein the estimated prediction mode deriving circuit is configured to derive the estimated prediction mode which is applicable to the target region based on a first prediction mode of the target region, wherein the estimated prediction mode of the target region is derived from the one or more first prediction modes when the set of prediction modes which includes the one or more first prediction modes is different from a set of prediction modes which is applicable to the target region and the set of prediction modes which is applicable to the target region includes one or more prediction modes not included in the set of prediction modes which includes the one or more first prediction modes.