A fluorine-containing compound gas, e.g., SF6 gas, is converted into a plasma and a silicon portion of an object to be processed is etched by the plasma. At the same time, using a light source having a peak intensity of light in a wavelength range of light absorption of a reaction product, e.g., SiF4, for which, to be more precise, ranges from 9 μm to 10 μm, the light is irradiated onto a surface of an object to be processed from the light source. The SiF4 molecules absorb the light, become activated and gain kinetic energy to be used in gaining an easy escape from a hole. As a consequence, an amount (a partial pressure) of fluorine radicals (F*) used as an etchant is increased and an etching rate of a silicon is increased.