1. An image coding method for coding an image on a block-by-block basis to generate a bitstream, the method comprising: determining a maximum number of merging candidates each of which is a combination of a prediction direction, a motion vector, and a reference picture index for use in coding of a current block; deriving a plurality of first merging candidates based on prediction directions, motion vectors, and reference picture indexes used in coding of blocks spatially or temporally neighboring the current block; determining whether or not a total number of the derived first merging candidates is smaller than the maximum number; deriving, by making a combination out of the derived first merging candidates, a second merging candidate for bi-directional prediction when it is determined that the total number of the derived first merging candidates is smaller than the maximum number; selecting a merging candidate to be used for the coding of the current block from the derived first merging candidates and the derived second merging candidate; and coding, using the determined maximum number, an index for identifying the selected merging candidate, and attaching the coded index to the bitstream.