A multi-display device for connecting projection screens of a plurality of image display devices so as to form one-screen image. Each of the image display devices includes a luminance lowering characteristic storage unit for storing a look-up table indicating average luminance lowering characteristic of a projection lamp, a timer unit for counting the lit time of lamp of the device, a lit time storage unit for accumulating and storing the lit time from the lamp use start moment, and an external communication unit for communicating.