A conductive shield board for protecting an aperture board from discharge is mounted between a positive electrode and said aperture board in an ink mist type high speed printer.The ink mist type printer operates on the principle that an ion stream modulated by an aperture board according to the pattern of the character to be printed, charges the ink mist, which is then attracted by an electric field, to the surface of the paper. However, a high potential on a positive electrode for generating said ion stream sometimes causes an undesirable discharge between the positive electrode and the aperture board.In order to avoid said undesirable discharge, a conductive shield board is inserted between the positive electrode and the aperture board.