In the case where a projector is used in combination with another projector, individual use of only one projector is enabled. A slave projector operates together with a master projector. In this case, the operation methods to start up the projector are a first method in which only a first key is pressed, a second method in which the first key and a second key are pressed simultaneously, and a third method in which the first key and a third key are pressed simultaneously. When the slave projector detects a user's operation with a remote controller, if the operation is done by the first method or the second method, the slave projector ignores this operation. Meanwhile, if the operation is done by the second method, the slave projector starts up the own device irrespective of the command from the master.