A control device that comprises: a communication management part configured to manage data communication using cyclic communication conforming to a predetermined communication cycle for a field network, wherein the control device is connectable to a slave device (211, 212, 213) via the field network, and the control device and the slave device are configured to perform communication of control data via the field network; a computation part configured to measure a propagation delay time from received data of data for actual measurement of propagation delay transmitted to the field network, and detect an operation state of the field network using a result of the propagation delay time; and a result output part configured to output a detection result of the computation part, wherein the computation part comprises: a propagation delay time measurement part configured to generate the data for actual measurement of propagation delay, to output the data for actual measurement of propagation delay to the slave device (211, 212, 213) via the communication management part, to acquire the received data of the data for actual measurement of propagation delay via the communication management part, and to calculate the propagation delay time of the slave device (211, 212, 213) from the received data of the data for actual measurement of propagation delay from the time stamp written to the received data; and a monitoring process execution part configured to perform monitoring of a bandwidth load of the field network from the propagation delay time, wherein the monitoring process execution part is configured to calculate the bandwidth load from a ratio of the propagation delay time of all frames transmitted within one cycle to the predetermined communication cycle. a communication management part configured to manage data communication using cyclic communication conforming to a predetermined communication cycle for a field network, wherein the control device is connectable to a slave device (211, 212, 213) via the field network, and the control device and the slave device are configured to perform communication of control data via the field network; a computation part configured to measure a propagation delay time from received data of data for actual measurement of propagation delay transmitted to the field network, and detect an operation state of the field network using a result of the propagation delay time; and a result output part configured to output a detection result of the computation part, wherein the computation part comprises: a propagation delay time measurement part configured to generate the data for actual measurement of propagation delay, to output the data for actual measurement of propagation delay to the slave device (211, 212, 213) via the communication management part, to acquire the received data of the data for actual measurement of propagation delay via the communication management part, and to calculate the propagation delay time of the slave device (211, 212, 213) from the received data of the data for actual measurement of propagation delay from the time stamp written to the received data; and a monitoring process execution part configured to perform monitoring of a bandwidth load of the field network from the propagation delay time, wherein the monitoring process execution part is configured to calculate the bandwidth load from a ratio of the propagation delay time of all frames transmitted within one cycle to the predetermined communication cycle. a propagation delay time measurement part configured to generate the data for actual measurement of propagation delay, to output the data for actual measurement of propagation delay to the slave device (211, 212, 213) via the communication management part, to acquire the received data of the data for actual measurement of propagation delay via the communication management part, and to calculate the propagation delay time of the slave device (211, 212, 213) from the received data of the data for actual measurement of propagation delay from the time stamp written to the received data; and a monitoring process execution part configured to perform monitoring of a bandwidth load of the field network from the propagation delay time, wherein the monitoring process execution part is configured to calculate the bandwidth load from a ratio of the propagation delay time of all frames transmitted within one cycle to the predetermined communication cycle.