1. A moving picture decoding device that decodes a bitstream obtained by coding moving pictures using inter-prediction based on inter-prediction information of a merge candidate in units of blocks obtained by partitioning each picture of the moving pictures, comprising: a prediction information decoding unit that decodes information indicating a previously designated number of merge candidates; a prediction information deriving unit that derives merge candidates from inter-prediction information of a prediction block neighboring to a decoding target prediction block or a prediction block present at the same position as or near the decoding target prediction block in a decoded picture at a temporally different position from the decoding target prediction block; a candidate list constructing unit that constructs a merge candidate list from the derived merge candidates; an additional merge candidate deriving unit that repeatedly performs a first process for adding to the merge candidate list a merge candidate of which the moving vector has a first value and the reference index has a second value; a valid merge candidate supplementing unit that repeatedly performs a second process for adding to the merge candidate list after the first process a merge candidate of which the motion vector has the first value and the reference index has a third value until the number of merge candidates included in the merge candidate list after the first process reaches the designated number of merge candidates when the number of merge candidates included in the merge candidate list after the first process is smaller than the designated number of merge candidates; and a motion-compensated prediction unit that selects one merge candidate from the merge candidates included in the merge candidate list of which the number of merge candidates reaches the designated number of merge candidates and performs inter-prediction on the decoding target prediction block using the inter-prediction information of the selected merge candidate, wherein: the first value is a predefined value that does not differ each time the first process and the second process occur; the second value is a variable that differs each time the first process occurs; and the third value is a predefined value that does not differ each time the second process occurs.