The InGaP solar cell having superior high radiation resistance is expected to be a powerful candidate for a dosimeter under high radiation dose rate environment. Because the minority carrier diffusion length (L) is a factor determining the solar cell performance and L changes due to radiation damage, it is important to predict the detector performance based on the relationship between L and the absorbed dose. In this study, the effect of L on a radiation-induced current as a dose signal in InGaP solar cell is clarified by irradiation tests and empirical calculations. In order to estimate the L for InGaP solar cell, measuring the short-circuit current density (JSC) as afunction of γ-ray dose rate is conducted. Based on the experimental results and the empiricalformula of the relationship between L and JSC, the operational lifetime of the InGaP solar cell detector under various dose rate is estimated and is determined by the cumulative dose. The present result suggests the InGaP solar cell has high potential as a radiation resistant dosimeter for contributing to the decommissioning of the Fukushima Daiichi nuclear power plant.