Laser processing method using plasma light detection for forming a pore in a substrate

A laser processing method for applying a laser beam to the reverse side of a substrate with a device formed on a face side thereof and including an electrode pad, to form a pore in the substrate that leads to the electrode pad, includes an irradiation area setting step of detecting the size of the electrode pad and setting an irradiation area for the laser beam such that the pore to be formed is positioned within the electrode pad. After the irradiation area setting step has been performed, the laser beam is applied to the reverse side of the substrate to form a pore in the substrate at a position corresponding to the electrode pad. First plasma light emitted from the substrate and second plasma light emitted from the electrode pad are detected. When the second plasma light is detected, the beam is stopped from being applied to the substrate.