To suppress wear of a driving shaft and a shaft sealing member which seals a gap between the driving shaft and a through-hole which the driving shaft penetrates.

A through-hole 91 which a driving shaft 88 penetrates is formed in a bottom plate of a cell 50. A housing chamber 93 is formed to be continued from the through-hole 91. A shaft sealing member 92 is attached in the housing room 93 to seal between the through-hole 91 and the driving shaft 88. The surface of the driving shaft 88 which is at least in contact with the shaft sealing member 92 is covered with a sleeve 119 comprising a ultra-high molecular weight polyethylene (UHMW-PE) having low friction performance and high wear resistance, thereby wear of the driving shaft 88 and the shaft sealing member 92 is suppressed.