A radiation imaging apparatus comprising: a pixel array in which a plurality of pixels are arrayed, each of the plurality of pixels including a converter (s) configured to accumulate charges corresponding to radiation; a bias line (Vs) configured to apply a bias potential to the converters (s) of the plurality of pixels; a detection circuit configured to detect a current flowing to the bias line (Vs); and a control means configured to control a detection operation of detecting a start of irradiation of the pixel array based on an output from the detection circuit, to control a charge accumulation operation of the plurality of pixels in accordance with the detection of the start of irradiation, and to control a readout operation of reading out a signal from the pixel array after the charge accumulation operation, wherein the detection circuit comprises a current-voltage conversion amplifier including an operational amplifier and a feedback path, the operational amplifier having a first input terminal to which a reference bias potential (Vs_ref) is applied, a second input terminal to which the bias line (Vs) is connected, and an output terminal, the feedback path being configured to connect the output terminal and the second input terminal, the detection circuit being configured to supply a potential corresponding to the reference bias potential (Vs_ref) to the bias line (Vs) from the output terminal via the feedback path, and the control means configured to control the impedance of the feedback path in the detection operation to be higher than the impedance of the feedback path in the readout operation. a pixel array in which a plurality of pixels are arrayed, each of the plurality of pixels including a converter (s) configured to accumulate charges corresponding to radiation; a bias line (Vs) configured to apply a bias potential to the converters (s) of the plurality of pixels; a detection circuit configured to detect a current flowing to the bias line (Vs); and a control means configured to control a detection operation of detecting a start of irradiation of the pixel array based on an output from the detection circuit, to control a charge accumulation operation of the plurality of pixels in accordance with the detection of the start of irradiation, and to control a readout operation of reading out a signal from the pixel array after the charge accumulation operation, wherein the detection circuit comprises a current-voltage conversion amplifier including an operational amplifier and a feedback path, the operational amplifier having a first input terminal to which a reference bias potential (Vs_ref) is applied, a second input terminal to which the bias line (Vs) is connected, and an output terminal, the feedback path being configured to connect the output terminal and the second input terminal, the detection circuit being configured to supply a potential corresponding to the reference bias potential (Vs_ref) to the bias line (Vs) from the output terminal via the feedback path, and the control means configured to control the impedance of the feedback path in the detection operation to be higher than the impedance of the feedback path in the readout operation.