PURPOSE: To prevent noises with a simple structure without increasing manhours for assemblying, by integrally incoporating together both electrically conductive cover for shielding a pick-up coil in a signal generator and resin cover for protecting from high voltage.

CONSTITUTION: A generator is attached with a resin cover having therein with fitting holes hooked onto bolts 6 on the generator to prevent the resin cover 1a from coming off. Further, the resin cover 1a is integrally incorporated with a copper cover 1b used as a filter for shielding a pick-up coil 2 against noises from the outside, in the position where the copper cover 1b shields the pick-up coil 2. With this arrangement, noises may be prevented with a simple structure without increasing manhours.