Accumulation of photo-induced carriers at the SiO₂/Si interface has been studied using a graphene device. In this device, CVD graphene is transferred on the SiO₂/Si substrate and serves as a charge sensor for detecting accumulation of the photo-induced carriers. Photo-excited carriers in the Si substrate are collected at the SiO₂/Si interface owing to a gate voltage applied to graphene and remain at the interface even after the light is turned off. The carrier accumulation mechanism is explained by band bending at the SiO₂/Si interface. The polarity of photo-induced carriers is controlled by that of the gate voltage.