A processing device according to one aspect of the present invention includes a provider configured to provide a process to a first device connected to a network of a process control system in a plant, an acquirer configured to acquire a live list generated by the first device based on diagnostic communication packets transmitted from the first device at predetermined time intervals, the live list comprising information indicating whether communication between the first device and another processing device that provides a process to the first device is norma, the processing device and the another processing device constituting a redundant pair 1, and a switch configured to switch a state of the provider from a standby state in which the provider is on standby to provide the process to the first device to an active state in which the provider provides the process to the first device based on the acquired live list.