A target is irradiated with a laser beam having an energy of h.nu./n, where n is an integer, instead of the photon energy h.nu. required for a reaction to proceed using a conventional single-photon excitation method. A desired reaction is selectively induced by the absorption of multiple photons instead of single photons. It is thus possible to induce a photoreaction in which an absorption is carried out in accordance with a selection theory different from that of single-photon absorption, and which is quite different from a photoreaction induced by a single-photon excitation. Namely, a target or reactant is irradiated with a photon energy h.nu..sub.2 having a wavelength integral times longer than that of the photon energy h.nu..sub.1 necessary for a photoreaction to proceed. Photon energy h.nu..sub.2 is either used in place of the photon energy h.nu..sub.1 necessary for a photoreaction, or together with the photon energy h.nu..sub.1, to thereby make the reactant absorb the energy equal to the photon energy h.nu..sub.1. The method is capable of selectively and preferentially inducing a specific photoreaction among various possible, concurrent photoreactions.