A picture coding device partitions differential information between a coding target picture and a prediction target picture into sub blocks and codes the partitioned sub blocks. The device codes significant sub block information that represents whether or not all the values of differential coefficients belonging to the sub block are zero and codes significant differential coefficient information that represents whether or not the value of the differential coefficient is zero. A context index processing unit derives a context used for coding the significant sub block information and a context used for coding the significant differential coefficient information of the differential coefficient that is a coding target based on an addition equation using the significant sub block information of a coded sub block that is horizontally neighboring to the coding target sub block and the significant sub block information of a coded sub block that is vertically neighboring.