Indium gallium phosphide (InGaP) solar cell is expected to be a dosimeter devise under a harsh radiation environment since it has high radiation resistance. InGaP solar cell is now considered to be one of strong candidates for a dosimeter to measure radiation level in the damaged reactors at Fukushima Daiichi Nuclear Power Station. Degradation characteristics of an InGaP solar cell against alfa-ray, which is one of the major radiation particles in the reactors, are obtained by 5-MeV helium-ion irradiation, and the degradation is analyzed using displacement damage dose.