1. An image decoding method comprising: assigning smaller first indices to pictures preceding a current picture in display order than first indices assigned to pictures following a current picture, the first indices associating blocks included in a current picture with each picture that is referred to by the blocks; assigning smaller second indices to pictures following a current picture in display order than second indices assigned to pictures preceding a current picture, the second indices associating blocks included in a current picture with each picture that is referred to by the blocks; selecting, as a first reference picture, a picture with the smallest first index out of a plurality of pictures that are referred to by a plurality of neighboring blocks, the plurality of neighboring blocks being at the neighborhood of a current block in a current picture; selecting, as a second reference picture, a picture with the smallest second index out of the plurality of pictures; and decoding the current block by using the first reference picture and the second reference picture.