Disclosed is a pH-sensitive block copolymer obtained by copolymerization of: a polyethylene glycol compound ; and at least one poly compound selected from the group consisting of a poly and poly or a copolymer thereof. The manager manages the storing of the coded partitions in the buffer whereby during the storing of the coded partitions if it is determined the buffer is full, a coded least perceptually significant partition currently stored in the buffer is overwritten by data from a coded more perceptually significant partition. Character string reverser reverses the alignment of the groups. An LDD is formed along the trench and has an impurity concentration that is lower than that of the source region and the drain region.