Provided are a communication control device and a communication system capable of detecting message transmission in the case where an invalid device transmits a message to a common communication line. A monitoring device decides a reference time point t0 for periodical message transmission by an ECU, decides multiple scheduled transmission time points t1, t2, . . . obtained by adding a period corresponding to an integer multiple of a transmission cycle T of a message to the reference time point t0, and decides that a predetermined period including each of the scheduled transmission time points is a permission period for message transmission. The monitoring device determines whether or not a detected message on a CAN bus has been transmitted during the permission period. If determined that transmission of an invalid message is not permitted, the monitoring device performs processing of causing the ECU which receives the message to discard the message.