A radiation image pickup apparatus includes a pixel array having pixels each including a conversion element and a switch element, a drive circuit for controlling the switch element between a conducting state and a non-conducting state, a detection unit for outputting a detection signal varying with the intensity of irradiation of the pixel array, and an arithmetic unit for calculating a start threshold value used to detect start of irradiation based on the signal output from the detection unit during a period when radiation is not emitted onto the pixel array in which the switch elements are sequentially set in a conducting state on a row-by-row basis by the drive circuit and the signal output from the detection unit during a period when radiation is emitted onto the pixel array in which the switch elements are sequentially set in a conducting state on a row-by-row basis by the drive circuit.