PURPOSE: To provide an infrared ray multiplex transmission system and infrared ray reception device which never leaks a transmitted signal to the outside from a specific area, has no possibility of interference with signals outside the area, and can increase the number of channels of transmission by multiplexing.

CONSTITUTION: Plural infrared ray transmission devices 1 multiplex multichannel digital signals by time division, make FSK modulation, and transmit the results as infrared ray signals. Each infrared ray transmission device make FSK modulation with a carrier frequency different from others, so that infrared ray reception device 30 receives the infrared ray signals which are frequency-multiplexed after time-division multiplexing. The multichannel digital signals are sent with the FSK-modulated infrared ray signals, so no sent signal leaks from the specific area and no interference with the outside is caused. Further, the frequency multiplexing is usable in combination with the time-division multiplexing, so the number of transmission channels can be increased.