A serializer is equipped with a plurality of input terminals into which a plurality of binary signals are input in parallel, and converts the plurality of input binary signals into serial binary signals and transmits the serial binary signals to an optical transmission module. One input terminal out of the plurality of input terminals is assigned as an input terminal for preventing bit continuation by inserting “1” signals or “0” signals into the serial binary signals so that a predetermined number of bits of the same value may not be inserted continuously. Due to this structure, bit continuation can be prevented even for a signal generating source with no coding function by a simple configuration without increasing cost and size.