A laser oscillator of a laser processing apparatus generates burst pulses each composed of a plurality of sub-pulses. The plurality of sub-pulses are generated in such a manner that the energy of the sub-pulse sequentially changes from a lower energy to a higher energy, and the burst pulses are applied to a wafer, whereby the wafer is formed therein with shield tunnels extending from the front surface to the back surface of the wafer and each being composed of a minute hole and an amorphous phase surrounding the minute hole.