To substantially attain high speed date transmission even when the speed limit of a device is present by providing a means for driving a light emitting element by a multilevel signal, and transmitting it as an optical signal, and a frequency correcting means for correcting the frequency characteristics of the light emitting element.

A free spatial light transmitter 1 receives a signal from an input terminal 10 with a buffer 11, and transmits this signal through a 4B5B encoder 12, a ternary encoding encoder 13, a pre-emphasizer 14, an LED driving circuit 15, and an LED 18 as a free spatial light 19. Then, when the frequency characteristics of the LED 18 are overlapped on an LED driving current spectrum flattened by the pre-emphasizer 14, the light of a spectrum almost equal to the original power spectrum of the ternary code is transmitted as a signal light. Thus, even when the speed limit of the device is present in the case of using the LED 18 whose modulating speed is relatively slow, high speed data transmission can be substantially attained.