A variable c1 is counted up by one for the Nth bit in attribute information=“1”, and a variable c0 is counted up by one for the Nth bit=“0” (S402). When the total code amount of the coding results of pieces of attribute information which have been coded up to now has exceeded a predetermined threshold, a value held by the variable c0 at this time and that held by the variable c1 are compared with each other to determine which of “1” or “0” is set for the Nth bit of subsequently input attribute information (S405).