A photonic band-gap fiber comprises a first core having a refractive index that is not higher than a refractive index of a clad; a second core that is disposed so as to surround the first core and has a refractive index that is lower than the refractive index of the first core; a clad that surrounds the second core; and a periodic structure portion that is disposed in the clad in a vicinity of the second core and is constituted by high-refractive index portions that have a refractive index higher than that of clad and form the periodic structure, and the periodic-structure portion functions as a wave-length filter. By the function of the periodic structure portion as a wave-length filter, it is possible to reduce the propagation loss of the transmission wavelength and increase the propagation loss of the cutoff wavelength.