We present an electron spin resonance (ESR) study of inverted-type polymer solar cells with the structure of indium tin oxide (ITO)/amorphous titanium oxide (TiOx)/regioregular poly(3-hexylthiophene) (P3HT):phenyl-C-61-butryic acid methyl ester (PCBM)/Au. Two light-induced ESR signals with narrow and broad linewidth were observed by light irradiation. We found a strong correlation between typical light-soaking effects and the ESR signal with the broad linewidth under UV light irradiation. From the fact that the light-soaking effect requires UV irradiation, the ESR signal with the broad linewidth is found to be related with the light-soaking effect.