PURPOSE: To control beam radiation by detecting that beam has passed through a printed wiring board to acquire a hole of a uniform diameter from a front to a rear when processing a through-hole by radiating energy beam of a laser to the printed wiring board.

CONSTITUTION: Pulses P1 to P8 are output from arm output means 6 by a pulse treatment means 14 and a through-hole is processed in a printed wiring board. After a hole penetrates, beam light is emitted through a through-hole and injected to a photosensitive element 16 which is arranged at a position to receive peripheral light flux of an optical axis of beam light. The photosensitive element 16 outputs electric signal on receiving incident light and inputs it to an amplifier 20. A signal from the amplifier 20 enters the pulse generation means and forbids pulse generation, thereby stopping laser output.