PURPOSE: To equalize allotment of voltage to each other without increasing a size of the whole arrester by using pure water as a refrigerant and cooling ZnO elements individually.

CONSTITUTION: A reactor winding 3a and thyristor 1 being main heating elements within a thyristor valve is cooled by a closed-loop circulation of water- cooled pure water through a heat sink 1a via a cooling insulation pipe 1b. Also, a portion of pure water is fed into an arrester accommodated within an insulating cylinder 12, in which ZnO elements and copper plates are laminated alternately, to cool the ZnO elements 5. Moreover, this apparatus is equipped with ion-exchange resin 6, water-circulating pump 7 and heat exchanger 8 for refrigeration and for keeping a purity of water. Thus, a rate of allotment of voltage between series ZnO elements can be improved.