A weighting factor mode determination unit determines whether to code an interlaced picture in a field mode or a frame mode, based on a value of a flag “AFF” indicating whether or not to switch between the field mode and the frame mode on a block-by-block basis and notifies switches and a multiplexing unit of the determined mode. The switches select either the field mode or the frame mode according to the notified mode. A field weighting factor coding unit or a frame weighting factor coding unit performs respectively the coding of respective weighting factors when the respective modes are selected.