1. A moving picture decoding device adapted to decode a bitstream in which moving pictures are coded in units of blocks obtained by partitioning each picture of the moving pictures, comprising: a first bitstream decoding unit configured to set the number of merge candidates; a second bitstream decoding unit configured to decode information indicating indices of the merge candidates; a spatial merge candidate generation unit configured to derive spatial merge candidates from a first predefined number of blocks neighboring a prediction block subject to decoding; a temporal merge candidate generation unit configured to derive a temporal merge candidate from a block that exists at the same position as or near a prediction block subject to decoding in a decoded picture that is different from the prediction block subject to decoding; a merge candidate addition unit configured to add the spatial merge candidates and the temporal merge candidates to a merge candidate list; a coding information selection unit configured to select a merge candidate from the merge candidates added to the merge list; and a motion compensation prediction unit configured to perform inter prediction of the prediction block subject to decoding by the merge candidate thus selected, wherein the second bitstream decoding unit derives the indices of the merge candidates based on the number of the merge candidates; and the spatial merge candidate generation unit stops deriving the spatial merge candidates when the number of the derived spatial merge candidates reaches a second predefined number smaller than the first predefined number.