A transportable electronic cassette and an image reading device carry out communication by laser light between themselves, with the separation distance to the casing of the opposing device detected by separation distance sensors provided respectively to the electronic cassette and the image reading device. The value of the separation distance detected is monitored to see whether or not the separation distance exceeds a reference value of the detected value at the start of communication by a specific value or more. Conclusion can be made that there has been a relatively large change in the relative position of the electronic cassette and the image reading device if the detected value of the separation distance becomes greater than the reference value by the specific value or more, and consequently emission is halted of the laser light from the electronic cassette and the image reading device.