Laser processing method

A laser processing method for applying a laser beam to a back surface of a substrate with a device formed on a front surface thereof and including an electrode pad, to form a fine hole in the substrate that reaches the electrode pad, the method comprising:

• a laser beam applying step of applying the laser beam to the back surface of the substrate to form a fine hole in the substrate at a position corresponding to the electrode pad;
• a detecting step of detecting, through a first optical path, first plasma light having a first wavelength emitted from the substrate at the same time that the fine hole is formed in the substrate by the laser beam applied thereto;
• and, through a second optical path, second plasma light having a second wavelength different from the first wavelength emitted from the electrode pad;
• and
• a laser beam irradiation finishing step of stopping application of the laser beam when the second plasma light is detected in the detecting step,
• wherein, in the laser beam applying step, a peak power density of the laser beam to be applied is set in a range from 175 GW/cm2 or less to 100 GW/cm2 or more.