A picture decoding device comprising: a bit strings decoding unit configured to obtain a bitstream including a flag indicating whether a motion compensation of an affine model is enabled; a spatial candidate derivation unit (321, 341) configured to derive a spatial candidate from inter prediction information of a block neighboring a decoding target block and register the derived spatial candidate as a candidate in a first candidate list; a history-based candidate derivation unit (323, 345) configured to generate a second candidate list by adding a history-based candidate included in a history-based candidate list as a candidate to the first candidate list; a candidate selection unit (327, 347) configured to select a selection candidate from candidates included in the second candidate list; and an inter prediction unit configured to perform inter prediction using the selection candidate, wherein the history-based candidate derivation unit switches between whether or not a history-based candidate overlapping a candidate included in the first candidate list is added in accordance with a prediction mode of the decoding target block, wherein the history-based candidate derivation unit adds the history-based candidate as a candidate to the first candidate list if a history-based candidate does not overlap a candidate included in the first candidate list when the prediction mode is merge mode, and adds the history-based candidate as a candidate to the first candidate list regardless of whether or not the history-based candidate overlaps a candidate included in the first candidate list when the prediction mode is motion vector prediction mode, and wherein an inter affine flag indicating whether to apply motion compensation of the affine model on the decoding target block is further included in the bitstream if the flag is true. a bit strings decoding unit configured to obtain a bitstream including a flag indicating whether a motion compensation of an affine model is enabled; a spatial candidate derivation unit (321, 341) configured to derive a spatial candidate from inter prediction information of a block neighboring a decoding target block and register the derived spatial candidate as a candidate in a first candidate list; a history-based candidate derivation unit (323, 345) configured to generate a second candidate list by adding a history-based candidate included in a history-based candidate list as a candidate to the first candidate list; a candidate selection unit (327, 347) configured to select a selection candidate from candidates included in the second candidate list; and an inter prediction unit configured to perform inter prediction using the selection candidate, wherein the history-based candidate derivation unit switches between whether or not a history-based candidate overlapping a candidate included in the first candidate list is added in accordance with a prediction mode of the decoding target block, wherein the history-based candidate derivation unit adds the history-based candidate as a candidate to the first candidate list if a history-based candidate does not overlap a candidate included in the first candidate list when the prediction mode is merge mode, and adds the history-based candidate as a candidate to the first candidate list regardless of whether or not the history-based candidate overlaps a candidate included in the first candidate list when the prediction mode is motion vector prediction mode, and wherein an inter affine flag indicating whether to apply motion compensation of the affine model on the decoding target block is further included in the bitstream if the flag is true. wherein the history-based candidate derivation unit adds the history-based candidate as a candidate to the first candidate list if a history-based candidate does not overlap a candidate included in the first candidate list when the prediction mode is merge mode, and adds the history-based candidate as a candidate to the first candidate list regardless of whether or not the history-based candidate overlaps a candidate included in the first candidate list when the prediction mode is motion vector prediction mode, and wherein an inter affine flag indicating whether to apply motion compensation of the affine model on the decoding target block is further included in the bitstream if the flag is true.