A first data taking section takes first detection results from a first wireless tag reader which detects the come and go of an electronic cassette into and out of a first service zone, and also takes second detection results from a second wireless tag reader which detects the come and go of an electronic cassette into and out of a second service zone. A first alert controller drives a first speaker to start an alert when the first alert controller determines on the basis of the first detection results that the electronic cassette has gone out the first service zone. After the start of alerting, the first alert controller stops driving the first speaker to interrupt the alert when first alert controller determines that the electronic cassette has come in either the first service zone or the second service zone.