A theft preventing system easy to operate and capable of implementing countermeasures to theft of a projection type image display device is provided. The projection type image display device is equipped with a unit for detecting a used condition of the device and a unit for registering a used condition of the device. By further equipping the device with a unit for inhibiting normal operations of the device in its used condition other than the registered condition, a simple and easy-to-operate unauthorized use preventing system can be realized and thus protection of the device against theft can be attained.