An optical filter has a first long-period grating and a second long-period grating formed in a unitary optical fiber. A difference is not less than 100 nm between a wavelength at which optical coupling is maximum between core-mode light and cladding-mode light of a predetermined mode number in the first long-period grating and a wavelength at which optical coupling is maximum between core-mode light and cladding-mode light of the same mode number as the cladding-mode light of the predetermined mode number, in the second long-period grating. This optical filter can be constructed in compact size and can readily implement a desired transmission characteristic.