PURPOSE: To obtain a data collision deciding device whose reliability is improved by suitably detecting only actual collision without being affected by noise or the like.

CONSTITUTION: Transmission data and reception data are compared by a data comparing means 1 and a detection signal indicating the difference of both data is detected by a collision deciding circuit means 3 for a monitoring period from a monitoring period deciding means 2 at every bit and when the detection signal is outputted for a prescribed time or above, it is decided that the data collide with each other and a collision signal CLO is outputted.