A laser processes a circuit board including at least two layers. The two layers are a conductive layer with a plurality of holes and an insulating layer. The conductive layer is disposed on a surface layer of the insulating layer which is to be processed by applying a laser beam to portions of the insulating layer corresponding to the holes of the conductive layer. The circuit board is irradiated with the laser beam, and a reflected laser beam is detected, whereby the laser processing of the insulating layer corresponding to an abnormal hole in the conductive layer is stopped, if the detected value of the reflected laser beam is an abnormal value differing from a desired value. Also, the insulating layer corresponding to the hole is processed when the detected value of the reflected light conforms to the desired value.