A pixel includes a conversion element detecting radiation, and a switch between the element and a signal line. A readout unit reads out a signal on the signal line. The readout unit includes a reset unit that resets a potential of the signal line. A period during which the readout unit reads out a signal on the signal line includes a first period during which the signal line is reset, and a signal on the signal line in a state that the switch is not turned on is read out, and a second period during which the signal line is reset, and a signal on the signal line due to the switch being turned on is read out. The processing unit calculates a difference between the signals read out in the second and first periods.