Abstract: According to a method and apparatus for inducing a photochemical reaction, a substrate is placed in a closed cell with a window for transmitting an ultraviolet beam therethrough and is located to oppose the window, a gas is filled to cause the phtoochemical reaction upon irradiation of the ultraviolet beam into the cell, ultraviolet pulses are repetitively controlled to emit at a predetermined intensity, an ultraviolet beam size and shape are adjusted in accordance with a size and a shape of an irradiation portion of the substrate, the repetition frequency of the ultraviolet beam is adjusted and set to effectively cause the photochemical reaction on the substrate in accordance with the adjusted ultraviolet beam size and shape, and the ultraviolet beam having the set beam size and shape at the repetition frequency is irradiated through the window of the cell.