To provide a control system, a monitoring method, and a program for achieving the detection of any failure by monitoring a control output data value for making each control unit control a control target unit without adding any hardware.

A control output for making an application A on a control unit 1-1 control a control target unit 2-1 is monitored by a monitor A on a control unit 1-2. In the same way, the control output by an application B on the control unit 1-2 is monitored by a monitor B on a control unit 1-3, and the control output by an application C on the control unit 1-3 is monitored by a monitor C on a control unit 1-4, and the control output by an application D on the control unit 1-4 is monitored by a monitor D on the control unit 1-1.