An interactive system comprising: a transmitter configured to transmit an optical signal; and a projector, the projector including: a signal-intensity setting unit in which intensity information of a synchronization signal is set; a signal-intensity control unit configured to control intensity of the synchronization signal on the basis of the setting of the signal-intensity setting unit; a synchronization-signal transmitting unit configured to transmit the synchronization signal; and an image pickup unit configured to pick up an image of the optical signal transmitted from the transmitter, and the transmitter including: a receiving unit configured to receive the synchronization signal; and an optical-signal transmitting unit configured to transmit the optical signal in synchronization with the synchronization signal received by the receiving unit; wherein the projector is arranged to determine whether another projector is present around the projector; and wherein the signal-intensity control unit is operable to reduce the intensity of the synchronization signal in the event that the projector determines that another projector is present around the projector. a transmitter configured to transmit an optical signal; and a projector, the projector including: a signal-intensity setting unit in which intensity information of a synchronization signal is set; a signal-intensity control unit configured to control intensity of the synchronization signal on the basis of the setting of the signal-intensity setting unit; a synchronization-signal transmitting unit configured to transmit the synchronization signal; and an image pickup unit configured to pick up an image of the optical signal transmitted from the transmitter, and the transmitter including: a receiving unit configured to receive the synchronization signal; and an optical-signal transmitting unit configured to transmit the optical signal in synchronization with the synchronization signal received by the receiving unit; wherein the projector is arranged to determine whether another projector is present around the projector; and wherein the signal-intensity control unit is operable to reduce the intensity of the synchronization signal in the event that the projector determines that another projector is present around the projector. a signal-intensity setting unit in which intensity information of a synchronization signal is set; a signal-intensity control unit configured to control intensity of the synchronization signal on the basis of the setting of the signal-intensity setting unit; a synchronization-signal transmitting unit configured to transmit the synchronization signal; and an image pickup unit configured to pick up an image of the optical signal transmitted from the transmitter, and the transmitter including: a receiving unit configured to receive the synchronization signal; and an optical-signal transmitting unit configured to transmit the optical signal in synchronization with the synchronization signal received by the receiving unit; wherein the projector is arranged to determine whether another projector is present around the projector; and wherein the signal-intensity control unit is operable to reduce the intensity of the synchronization signal in the event that the projector determines that another projector is present around the projector. a receiving unit configured to receive the synchronization signal; and an optical-signal transmitting unit configured to transmit the optical signal in synchronization with the synchronization signal received by the receiving unit; wherein the projector is arranged to determine whether another projector is present around the projector; and wherein the signal-intensity control unit is operable to reduce the intensity of the synchronization signal in the event that the projector determines that another projector is present around the projector.