The present invention relates to a signal shaping circuit which is capable of shaping a returning light signal of a modulated signal whose signal level and signal length are arbitrarily varied such that it can be demodulated. The shaped signal pattern generation circuit 15 has, in advance, variation pattern information with regard to a predetermined variation pattern to be applied on a returning light signal recorded therein. The counter 13 inputs a returning light signal, counts a signal length thereof and sends a count value to the shaped signal pattern generation circuit 15. An A/D conversion 14 detects a signal level of the returning light signal and sends the detected signal level to the shaped signal pattern generation circuit 15 in a similar way. The shaped signal pattern generation circuit 15 refers to the variation pattern information and calculates an optimum signal pattern corresponding to a signal length and a signal level of the inputted, returning light signal. The calculated shaped signal pattern is outputted to a shaped signal generation circuit 16. Based on the signal pattern, the shaped signal generation circuit 16 converts the returning light signal and generates an optimum shaped signal.