To suppress deterioration or flow loss of grease charged into a chain wheel by reducing intrusion of wind and rain or radiation of sunshine to the inside of the chain cover.

An eaves part for thin plate-like opening part (26) extending toward a discharge electrode hammering speed reducer side is provided at an opening part (24) formed at the chain cover for penetrating a rotary shaft of the speed reducer therethrough. For example, the opening part eaves comprises a strip-like eaves part (26a) and a plurality of leg parts (26b) juxtaposedly extending from one long side of the eaves part. The eaves part has prescribed flexibility, and the leg part is magnetized. The eaves part is folded to form a right angle in relation to the leg part, and magnetized and curved along the form of the opening part and attached to the opening part.