A portable X-ray imaging apparatus includes an electronic cassette for imaging of an X-ray image and transmitting the X-ray image wirelessly. An image receiving unit communicates with the electronic cassette and receives the X-ray image wirelessly. A passive monitoring device receives ambient electromagnetic waves to measure communication environment by passive monitoring. An active monitoring device performs data communication between the electronic cassette and the image receiving unit to measure the communication environment by active monitoring. Furthermore, an evaluation device detects a cause of a communication failure by considering a result of the passive monitoring and a result of the active monitoring. The evaluation device detects that the cause of the communication failure is a blocking object blocking radio waves or a relative position between the electronic cassette and the image receiving unit according to the results of the passive and active monitoring.