1. A solid-state imaging device having a unit pixel arranged therein, the unit pixel comprising a photoelectric conversion part, a floating diffusion part, a transfer transistor configured to transfer a charge from the photoelectric conversion part to the floating diffusion part, a first reset transistor configured to reset the floating diffusion part, a charge storage capacitor, a charging transistor configured to charge the charge storage capacitor by a current corresponding to a charge in the floating diffusion part, a second reset transistor configured to reset the charge storage capacitor, an amplifying transistor configured to output an electric signal corresponding to a charge in the charge storage capacitor, and a selection transistor configured to selectively cause the amplifying transistor to be in an operation state.