A laser processed hole is formed in a workpiece. The workpiece has a first member formed of a first material bonded to a second member formed of a second material. The laser processed hole extends through the first member to the second member. The wavelength of plasma light generated by applying a pulsed laser beam to the first member and the second member is detected. Application of the laser beam is continued at a first power until the plasma light intensity generated from only the first member is decreased to reach a predetermined value. The laser beam is applied at a second power which is lower than the first power so as to not generate cracks in the first member when the plasma light intensity has reached the predetermined value. Application of the plasma laser beam stops when plasma light generated from the second member is detected.