1. A picture coding and decoding system which includes a picture coding apparatus that codes a current block included in a current picture to be coded and a picture decoding apparatus that decodes coded data of a current block included in a current picture to be decoded, wherein the picture coding apparatus includes: a referring unit: operable to refer to a first list in which smaller indices are assigned to pictures preceding a current picture in display order than indices assigned to pictures following the current picture, the first list being a list of index associating blocks included in the current picture with each picture that is referred to by the blocks, and operable to refer to a second list in which smaller indices are assigned to pictures following the current picture in display order than indices assigned to pictures preceding the current picture, the second list being a list of index associating blocks included in the current picture with each picture that is referred to by the blocks; a selecting unit: operable to select, from the first list, as a first reference picture, a picture with the smallest index out of a plurality of pictures that are referred to by a plurality of neighboring blocks, the plurality of neighboring blocks being located at the neighborhood of a current block in the current picture to be coded, and operable to select, from the second list, as a second reference picture, a picture with the smallest index out of the plurality of pictures; and a coding unit operable to predictive-code the current block by referring to the first reference picture and the second reference picture, wherein the picture decoding apparatus includes: a referring unit: operable to refer to a first list in which smaller indices are assigned to pictures preceding a current picture in display order than indices assigned to pictures following the current picture, the first list being a list of index associating blocks included in the current picture with each picture that is referred to by the blocks, and operable to refer to a second list in which smaller indices are assigned to pictures following the current picture in display order than indices assigned to pictures preceding the current picture, the second list being a list of index associating blocks included in the current picture with each picture that is referred to by the blocks; a selecting unit: operable to select, from the first list, as a first reference picture, a picture with the smallest index out of a plurality of pictures that are referred to by a plurality of neighboring blocks, the plurality of neighboring blocks being located at the neighborhood of a current block in the current picture to be decoded; operable to select, from the second list, as a second reference picture, a picture with the smallest index out of the plurality of pictures; and a decoding unit operable to decode the current block by referring to the first reference picture and the second reference picture.