A signal relay device comprises a first connection I/F, to which an emission switch is connected, a second connection I/F, to which an emission signal I/F of an electronic cassette is connected, and a third connection I/F, to which a switch I/F of a source control device is connected, and a signal processing unit. The signal processing unit generates an emission execution signal during a time period in which an emission command signal from the emission switch and an emission enable signal from the electronic cassette are inputted. The source control device drives an X-ray tube to allow X-ray emission while the emission execution signal is inputted.