An LED illumination device that can be mounted between a pair of sockets provided to a fluorescent light fixture is provided with a first power source circuit that illuminates an LED using DC power obtained by converting and rectifying AC power supplied from the sockets, and a second power source circuit that illuminates the LED using an internal battery. An IC chip installed in the illumination device detects a first current conduction state in two AC power lines that supply the AC power necessary to drive the first power supply circuit and a second current conduction state in another AC power line, and, on the basis thereof, determines whether the device is in a normal illumination mode, a normal off mode, or an emergency illumination mode. On the basis of the determination results, the LED illumination device is controlled according to the mode. When in the emergency illumination mode, the LED is illuminated by the second power source circuit, and the LED illumination device functions as an emergency light.