A terahertz band optical filter having a dielectric multilayer periodic structure in which a plurality of dielectric materials are periodically layered. Multi-cavity layers each having an optical path length of n times .lamda./2 (n is an integer greater than or equal to 1) and made of a low-refractive index medium are arranged. The cavity layers are coupled using a single-layer coupling layer having an optical path length .lamda./4 and made of a high-refractive index medium, thus forming a multi-cavity structure. Matching layers each including a high refractive index layer and a low refractive index layer each having an optical path length of .lamda./4 are disposed at either end of the multi-cavity structure.