PURPOSE: To eliminate the need for the use of a broad band differential amplifier for lots of split terminals and to reduce number of the broad band differential amplifiers by reproducing data signal from common terminals of a few number of split photodetectors.

CONSTITUTION: Broad band differential amplifiers 25, 26 set a level of common terminals 1A, 2A of split photodetectors 1, 2 to a voltage of a power supply 23 and use a feedback resistor R, through which a current flowing to the common terminals 1A, 2A is converted into a voltage. Low speed differential amplifiers 27 - 30 set a voltage of a 2nd power supply 24 to split terminals 1aB, 1bB, 2aB, 2bB of the split photodetectors 1, 2 and use a feedback resistor (r) by which a flowing current to the split terminals 1aB, 1bB, 2aB, 2bB into a voltage. Then a data signal is reproduced from common terminals 1A, 2A of a few number of the split photodetectors 1, 2. Thus, it is not use broad band differential amplifiers for lots of the split terminals 1aB, 1bB, 2aB, 2bB and number of the broad band differential amplifiers is decreased.