When operation keys are operated, a light-emitting device outputs an infrared signal corresponding to the operated operation keys. The infrared signal is applied to a light-detecting device. In response to the applied infrared signal, the light-detecting device generates a detected signal and supplies the detected signal to an amplifying circuit. The amplified detected signal from the amplifying circuit is decoded by a decoding circuit into a data code, which is supplied through an interface circuit to a computer. Based on control data supplied as the data code to the computer, the computer controls a projector to perform a process of displaying images page by page, for example.