The temperature dependency of an EDFA is compensated in a wavelength of 1570 to 1600 nm to realize a wavelength division multiplexing transmission, First and second long period gratings different in grating period from each other are formed in an optical fiber. The peak wavelength of the waveform of the light transmission loss characteristic of the first long period grating is formed on the shorter wavelength side than a transmission band. The peak wavelength of the waveform of the light transmission loss characteristic of the second long period grating is formed on the longer wavelength side than a transmission band. The wave forms of the above respective light transmission loss characteristics and the peak wavelengths of the wave forms are shifted depending on the temperature, and the light transmission loss value on the shorter wavelength side of the transmission band is increased as the temperature becomes higher, and the light transmission loss value on 'the longer wavelength side of the transmission band increases as the temperature becomes lower, to thereby compensate the temperature dependent gain characteristic of the optical amplifier in the gain compensation band on the shorter wavelength side and the longer wavelength side of the transmission band.
