To provide a communication system capable of rapidly checking the setting and connection of a communication module without complicating the system.

A transmission means 22 transmits a function (a master module/a slave module) of a communication module 2 recognized on the basis of the setting of an internal switch 21 to the communication module 3 through a relay bus 1. A collating means 23 collates the function (the master module/the slave module) transmitted from the communication module 3 and the function of the communication module 2 recognized by itself. A stop means 24 stops the data output from the communication module 2 according to the result in the collating means 23.