PURPOSE: To obtain an excellent code error rate characteristic over a wide dynamic range while increasing difference in detectable collision signal level, by providing two discriminators for data reproduction and signal collision detection.

CONSTITUTION: A light signal 11 inputted to a receiver is converted into a voltage signal by a photodetecting element 12 and a preamplifier 13, and the voltage signal is branched to a discriminator 16 for data reproduction and a discriminator 20 for signal collision detection. A digital signal obtained by the discriminator 16 is converted from a transmission line code to an original code and outputted as reproduced data. A digital signal obtained by the discriminator 20, on the other hand, is supplied to a code rule checking circuit 23 to obtain code rule contravention information, which is transmitted to a signal collision decision circuit 24 to output a signal collision detection signal according to the degree of the contravention.