Superluminescent diode (SLD) for the light source of fiber-optic gyroscope (FOG) will degrade along with the operating time elapse. Operating lifetime of fine SLDs has been estimated from the results of accelerated aging carried out for 3500 h at the ambient temperature of 373K and 358K respectively and the aging rate and active energy were estimated from power-time (P-t) curve fitting. The value of active energy is determined to be 0.82 eV and mean lifetime in practical operation at driving current of 100 mA and an ambient temperature of 298 K is over 106 h, and high reliability and long lifetime required for light source of FOG are confirmed. The degrading mechanism of 1300 nm SLDs was analyzed for developing high reliability devices.