A coding device and a decoding device are configured to include an estimated prediction mode deciding section (122a) including: a reference block selecting section (1223a) for selecting a reference block for use in estimating an intra-prediction mode for a subject block; an estimating number deciding section (1222a) for deciding an estimating number of estimated values to be estimated for the intra-prediction mode; and an estimated prediction mode deriving section (1221a) for estimating, based on a reference block, the estimating number of estimated values of the prediction mode.