1. A picture decoding device adapted to decode a bitstream coded in units of blocks obtained by partitioning each picture of moving pictures, by using motion vectors, comprising: a bitstream decoding unit that decodes the bitstream to derive, for each block subject to decoding, prediction mode information indicating an inter prediction mode and a first index, or derives prediction mode information indicating an inter prediction mode, a second index, and a motion vector difference; an acquisition unit that acquires coding information on a plurality of reference blocks for the block subject to decoding; a first mode output unit that constructs, when the inter prediction mode indicated by the prediction mode information decoded by the bitstream decoding unit indicates a first inter prediction mode that uses information on a motion vector provided in the coding information of the reference block identified by the index, a first candidate list by deriving the coding information from the plurality of reference blocks in accordance with a first predetermined order, identifies a first candidate in the first candidate list by referring to the first index decoded by the bitstream decoding unit, and outputs the coding information on the first candidate; a second mode output unit that constructs, when the inter prediction mode indicated by the prediction mode information decoded by the bitstream decoding unit indicates a second inter prediction mode that uses a motion vector difference associated with the reference block identified by the index, a second candidate list by deriving motion vector predictors from the plurality of reference blocks in accordance with a second predetermined order, identifies a second candidate in the second candidate list by referring to the second index decoded by the bitstream decoding unit, and derives and outputs a motion vector for the block subject to decoding from the motion vector predictor of the second candidate and from the motion vector difference decoded by the bitstream decoding unit; and a motion compensation prediction unit that performs motion compensation using the inter prediction mode indicated by the prediction mode information decoded by the bitstream decoding unit, by referring to information output from the first mode output unit or the second mode output unit, so as to generate a prediction picture for the block subject to decoding, wherein the first and second mode output units use a plurality of common blocks as the plurality of reference blocks, wherein the first predetermined order for constructing the first candidate list in the first mode output unit differs from the second predetermined order for constructing the second candidate list in the second mode output unit, wherein the plurality of reference blocks include decoded blocks spatially neighboring the block subject to decoding and blocks included in a picture at a different point of time from a picture including the block subject to decoding, wherein the first mode output unit derives the coding information from the plurality of reference blocks such that a position of a block included in a picture at a different point of time in the first predetermined order is lower than a position of a spatially neighboring block in the first predetermined order, wherein the first mode output unit constructs the first candidate list from a plurality of decoded blocks spatially neighboring the block subject to coding in accordance with the first predetermined order, by providing a predetermined upper limit to the number of spatially neighboring decoded blocks arranged in the first candidate list, and by using as many reference blocks as defined by the upper limit in the first predetermined order, wherein a spatially neighboring left candidate block is at the top of the first predetermined order, followed by a candidate block above, a candidate block to the upper right, a candidate block to the lower left, a candidate block to the upper left, and a candidate block located in another picture at a different point of time, and wherein the second mode output unit constructs the second candidate list, by defining those of the plurality of spatially neighboring decoded blocks arranged to the left of the block subject to decoding as belonging to a block group to the left, defines those blocks arranged above the block subject to decoding as belonging to a block group above, derives a candidate representing each of the block groups, and constructs the second candidate list by defining the second predetermined order such that the candidate from the block group to the left and the candidate from the block group above are arranged in the stated order.