A radiation imaging apparatus comprising a detection unit arranged to detect radiation from an irradiation unit arranged to irradiate with the radiation; a reception unit configured to receive, from an external apparatus, irradiation information about an irradiation time at which irradiation of the radiation is to be started; a control unit configured to control the detection unit so that the detection unit is ready to detect the radiation at the irradiation time, based on the irradiation information; and a transmission unit configured to transmit photographing permission information, which indicates that the detection unit is ready to detect the radiation at the irradiation time, to the external apparatus if the detection unit is expected to be ready to detect the radiation at the irradiation time.