PURPOSE: To enable fast dry etching even by low energy by directly forming a deposit layer acquired by decomposing and polymerizing an etching gas prior to etching on the surface of a semiconductor substrate or the surface of a light transmitting substrate oppositely faced to said substrate.

CONSTITUTION: NH3 gas is caused to flow into an etching chamber 1 while an silicon substrate 3 is irradiated by a laser. NH3 molecules absorb the laser and decompose, and an silicon nitride film is shaped onto the surface of the silicon substrate. A gas in the chamber is replaced with SF6 gas. The polymerization product 3a of SF6 gas is deposited on the surface of the silicon nitride film through glow discharge or microwave discharge. The silicon substrate 3 on which a reaction product is deposited is irradiated by the laser from a light-introducing window 4. The nitride film irradiated by the laser is optically pumped and the temperature of the nitride film is elevated, SF6 gas and the polymerization product are excited and decomposed near the nitride film, and fluorine atoms abounding in reactivity are generated and the silicon nitride film is etched.