1. A decoding method comprising: receiving encoded data, the encoded data including: a syntax describing a plurality of parameters that are obtained by using a common index commonly used for a first reference image list and a second reference image list, and a syntax describing a syntax element including a maximum number of values taken by the common index; regenerating the syntax element from the encoded data; obtaining the plurality of parameters using the regenerated syntax element and the common index; and performing a decoding process that uses a reference image and the plurality of parameters that are obtained, wherein the common index is used to obtain the reference image from the first reference image list and the second reference image list.