A ticket showing contents utilization right is issued separately form contents to permit exchange of the ticket between terminals. These masks comprise a lower mirror covering a substrate, and two types of reflecting zones Z1 and Z2 in order to form a phase shift mask. A second switching time is calculated in a form of a time which is a subtraction of the first switching time from a half period of a carrier and, using this time, control signals to switch on and off of the switching elements are generated.