To provide an automatic contrast agent-injecting device with high efficiency in the use in total, capable of achieving reliable and inexpensive radiography with a contrast agent in which a radiation irradiation device, a flat surface detector and the automatic contrast injection device are combined and associated with one another, and being used in other control modes than the associated mode.

The automatic contrast agent-injecting device has a plurality of control modes including the associated mode in which the timing for injecting the contrast agent, the timing for obtaining an X-ray image from the FPD 3 and the timing for the X-ray irradiation device to apply X-rays are associated. The automatic contrast agent-injecting device comprises a control part 15 for controlling the injection of the contrast agent by a contrast agent injection means in a control mode selected from the plurality of control modes, and a determination means for determining whether the control by the control part 15 in the associated mode is possible or not. The associated mode can be selected only when the determination means determines that the control in the associated mode is possible.