A radiation detector comprising: an active pixel that comprises a sensor element configured to cause an electrical signal to be generated based on a radiation dose of incident radiation; an amplifying transistor configured to amplify the electrical signal, and a reset transistor configured to reset a gate electrode of the amplifying transistor to an initial voltage, if the reset transistor is switched on; the radiation detector being configured to read a current value that flows between a source electrode and a drain electrode of the amplifying transistor based on a change in voltage of the gate electrode of the amplifying transistor, the change being caused by the electrical signal that is provided from an output of the sensor element to the gate of the amplifying transistor; and a reset reading circuit that includes an amplifier and is configured to read the current value using the amplifier, and that includes an initial value determination circuit comprising a resistance connected on one side to the drain electrode of the amplifying transistor and a minus side input terminal of the amplifier, and connected on its other side to ground; wherein a source electrode of the reset transistor is connected to an output terminal of the amplifier, a drain electrode of the reset transistor is connected to the output of the sensor element and the gate electrode of the amplifying transistor, and a gate electrode of the reset transistor is connected to a reset signal line; the output of the amplifier is fed back to the minus side input terminal of the amplifier via a feedback capacitance connected between the minus side input terminal of the amplifier and the output terminal of the amplifier. and a reference voltage (Vint_b) is input to a plus side input terminal of the amplifier; the reset reading circuit is configured, if the reset transistor is switched on, to output the initial voltage to the gate electrode of the amplifying transistor via the source electrode and the drain electrode of the reset transistor so that the current value becomes a predetermined value that equals the quotient of the reference voltage (Vint_b) and the resistance. an active pixel that comprises a sensor element configured to cause an electrical signal to be generated based on a radiation dose of incident radiation; an amplifying transistor configured to amplify the electrical signal, and a reset transistor configured to reset a gate electrode of the amplifying transistor to an initial voltage, if the reset transistor is switched on; a sensor element configured to cause an electrical signal to be generated based on a radiation dose of incident radiation; an amplifying transistor configured to amplify the electrical signal, and a reset transistor configured to reset a gate electrode of the amplifying transistor to an initial voltage, if the reset transistor is switched on; the radiation detector being configured to read a current value that flows between a source electrode and a drain electrode of the amplifying transistor based on a change in voltage of the gate electrode of the amplifying transistor, the change being caused by the electrical signal that is provided from an output of the sensor element to the gate of the amplifying transistor; and a reset reading circuit that includes an amplifier and is configured to read the current value using the amplifier, and that includes an initial value determination circuit comprising a resistance connected on one side to the drain electrode of the amplifying transistor and a minus side input terminal of the amplifier, and connected on its other side to ground; wherein a source electrode of the reset transistor is connected to an output terminal of the amplifier, a drain electrode of the reset transistor is connected to the output of the sensor element and the gate electrode of the amplifying transistor, and a gate electrode of the reset transistor is connected to a reset signal line; the output of the amplifier is fed back to the minus side input terminal of the amplifier via a feedback capacitance connected between the minus side input terminal of the amplifier and the output terminal of the amplifier. and a reference voltage (Vint_b) is input to a plus side input terminal of the amplifier; the reset reading circuit is configured, if the reset transistor is switched on, to output the initial voltage to the gate electrode of the amplifying transistor via the source electrode and the drain electrode of the reset transistor so that the current value becomes a predetermined value that equals the quotient of the reference voltage (Vint_b) and the resistance.