The present invention addresses the problem of improving data transmission/reception equipment and transmission efficiency per hour. A light output unit 111 of a light transmitting/receiving device outputs n optical signals (where n is an integer value of 2 or more), each with a different wavelength, based on n respective pieces of data to be transmitted. From the n optical signals, a multiplexer 112 produces an optical signal multiplexed with respect to wavelength. From the multiplexed optical signal, an encryption unit 113 produces an encrypted signal by performing multi-level modulation on the basis of a Y-00 protocol. The problem is solved thereby.