A motion vector determining method for determining two motion vectors of a current block included in a current picture and determining two reference pictures which are selected from among a plurality of candidate reference pictures for the current picture, said motion vector determining method comprising: a co-located block specifying step of specifying a co-located block included in a first picture, said first picture being one of the plurality of candidate reference pictures that is different from the current picture, and located immediately after the current picture in display order, and said co-located block being a block located identically to the current block; a motion vector setting step of (i) setting the two motion vectors of the current block by referring to one motion vector used for motion-compensation of the co-located block in the case where the co-located block is an inter-picture coded block that is coded using only the one motion vector and one reference picture, and (ii) setting the two motion vectors of the current block to have zero-values in the case where the co-located block is an intra-picture coded block that is coded without using a motion vector; the method being characterized by a reference picture selecting step of (i) selecting, as one of the two reference pictures, a picture that is used as a reference picture in performing motion compensation on the co-located block and selecting, as the other of the two reference pictures, the first picture that is selected from among the plurality of candidate reference pictures for the current picture in the case where the co-located block is an inter-picture coded block that is coded using only the one motion vector and one reference picture, and (ii) selecting, as one of the two reference pictures, a second picture which is one of the plurality of candidate reference pictures for the current picture and located immediately before the current picture in display order and selecting, as the other of the two reference pictures, the first picture in the case where the co-located block is an intra-picture coded block that is coded without using a motion vector; and a motion compensation step of performing motion compensation on the current block by using the two motion vectors set in the motion vector setting step and the two reference pictures selected in the reference picture selecting step. a co-located block specifying step of specifying a co-located block included in a first picture, said first picture being one of the plurality of candidate reference pictures that is different from the current picture, and located immediately after the current picture in display order, and said co-located block being a block located identically to the current block; a motion vector setting step of (i) setting the two motion vectors of the current block by referring to one motion vector used for motion-compensation of the co-located block in the case where the co-located block is an inter-picture coded block that is coded using only the one motion vector and one reference picture, and (ii) setting the two motion vectors of the current block to have zero-values in the case where the co-located block is an intra-picture coded block that is coded without using a motion vector; the method being characterized by a reference picture selecting step of (i) selecting, as one of the two reference pictures, a picture that is used as a reference picture in performing motion compensation on the co-located block and selecting, as the other of the two reference pictures, the first picture that is selected from among the plurality of candidate reference pictures for the current picture in the case where the co-located block is an inter-picture coded block that is coded using only the one motion vector and one reference picture, and (ii) selecting, as one of the two reference pictures, a second picture which is one of the plurality of candidate reference pictures for the current picture and located immediately before the current picture in display order and selecting, as the other of the two reference pictures, the first picture in the case where the co-located block is an intra-picture coded block that is coded without using a motion vector; and a motion compensation step of performing motion compensation on the current block by using the two motion vectors set in the motion vector setting step and the two reference pictures selected in the reference picture selecting step.