WE CLAIM:
1. An image decoding method, characterized by :
assigning first indices to pictures preceding a current picture in display order which are smaller than first indices assigned to pictures following the current picture, the first indices associating blocks included in the current picture with each picture that is referred to by the blocks;
assigning second indices to pictures following the current picture in display order which are smaller than second indices assigned to pictures preceding the current picture, the second indices associating blocks included in the current picture with each picture that is referred to by the blocks;
selecting a picture with the smallest first index of the pictures that are referred to by neighboring blocks as a first reference picture, the neighboring blocks being at the neighborhood of a current block in the current picture;
selecting a picture with the smallest second index of the pictures that are referred to by neighboring blocks as a second reference picture;
decoding the current block by using at least the first reference picture and the second reference picture.
2. The image decoding method as claimed in claim 1, comprising determining, when
the neighboring blocks have only one motion vector referring to the first reference
picture, the motion vector as a first motion vector; and
determining, when the neighboring blocks have only one motion vector referring to the second reference picture, the motion vector as a second motion vector;
wherein, in the decoding, the current block is decoded with the first motion vector and the second motion vector.
3. An image decoding device comprising motion compensation decoding units (604,
705), characterized by :
an index assigning unit (604, 705) configured to assign first indices to pictures preceding a current picture in display order which are smaller than first indices assigned

to pictures following the current picture, the first indices associating blocks included in the current picture with each picture that is referred to by the blocks; and to assign second indices to pictures following the current picture in display order which are smaller than second indices assigned to pictures preceding the current picture, the second indices associating blocks included in the current picture with each picture that is referred to by the blocks;
a picture selecting unit (604, 705) configured to select a picture with the smallest first index of the pictures that are referred to by neighboring blocks as a first reference picture, the neighboring blocks being at the neighborhood of a current block in the current picture; and to select a picture with the smallest second index of the pictures that are referred to by neighboring blocks as a second reference picture; and
a decoding unit (604, 705) configured to decode the current block by using at least the first reference picture and the second reference picture.
4. The image decoding device as claimed in claim 3,
comprising a motion vector determining unit (604, 705), wherein,
when the neighboring blocks have only one motion vector referring to the first
reference picture, the motion vector determining unit is configured to determine the
motion vector as a first motion vector; and,
when the neighboring blocks have only one motion vector referring to the second
reference picture, the motion vector determining unit is configured to determine the
motion vector as a second motion vector; and wherein
the decoding unit is operable to decode the current block with the first motion
vector and the second motion vector.