An optical switch for switching signal light by allowing the signal light to obliquely enter a non-linear optical thin film 1 containing nano-crystal of metal oxide from a light path disposed in an optical material, and irradiating visible excitation light to the non-linear optical thin film to induce a total reflection phenomenon, thereby controlling the reflection and transmission behavior of the signal light, wherein a difference between a temperature coefficient factor of an optical index of the non-linear optical thin film and that of an optical index of the optical material which is in contact with the non-linear optical thin film is determined to be 15.times.10.sup.-6/.degree. C. or below. Preferably, it is determined to be zero or substantially zero. Thus, an influence due to the temperature increase is cancelled, and a malfunction of switching can be avoided.