1. An image encoding method for generating a bitstream by encoding an image on a block-by-block basis, the method comprising: dividing a current block that is a unit of encoding into a plurality of sub-blocks each of which is a unit of prediction; deriving, for each of the plurality of sub-blocks, one or more prediction information candidates each of which is a candidate for prediction information indicating a reference picture list, a motion vector, and a reference picture index; selecting a prediction information candidate from among the derived one or more prediction information candidates; and adding, to the bitstream, an index for identifying the selected prediction information candidate, wherein the deriving includes: determining whether or not a neighboring block neighboring each of the plurality of sub-blocks is included in the current block, and when the neighboring block is not included in the current block, determining the neighboring block to be a reference block available to the sub-block, and when the neighboring block is included in the current block, determining the neighboring block not to be the reference block; and deriving a prediction information candidate of the sub-block from prediction information of the reference block; and when a total number of the prediction information candidates is smaller than a predetermined number, generating one or more new candidates without using the prediction information of the reference block until the total number of the prediction information candidates reaches the predetermined number.