An input-output apparatus which is configured to be used in a process control system, comprising: a first interface to which a field device installed in a plant is connectable; a second interface to which a controller controlling the field device is connectable; a third interface to which an external setting apparatus is connectable, the third interface being a maintenance port; a switcher which is configured to switch between a connection of the second interface to the first interface and a connection of the maintenance port to the first interface; a control device which is configured to control the switcher; and a switch which is configured to be controlled by the control device and is disposed between the maintenance port and the control device, characterized in that the control device is configured to cause the input-output apparatus to enter to one of an online state in which the second interface is connected to the first interface by the switcher and the maintenance port is connected to the control device by the switch, a maintenance state in which the maintenance port is connected to the first interface by the switcher and the maintenance port is connected to the control device by the switch, and a maintenance disable state in which the second interface is connected to the first interface by the switcher but the maintenance port is disconnected from the control device by the switch, wherein the input-output apparatus further comprises: an engineering mode for engineering of the process control system allowing the input-output apparatus, when the setting apparatus sends a request for connection to the control device via the maintenance port, to transit from the online state to the maintenance state for settings and/or adjustments on the field device and on the input-output apparatus and allowing the input-output apparatus to transit back to the online state when the settings and/or adjustments are completed; and an operating mode for performing process control of the process control system when the input-output apparatus is in the maintenance disable state. a first interface to which a field device installed in a plant is connectable; a second interface to which a controller controlling the field device is connectable; a third interface to which an external setting apparatus is connectable, the third interface being a maintenance port; a switcher which is configured to switch between a connection of the second interface to the first interface and a connection of the maintenance port to the first interface; a control device which is configured to control the switcher; and a switch which is configured to be controlled by the control device and is disposed between the maintenance port and the control device, characterized in that the control device is configured to cause the input-output apparatus to enter to one of an online state in which the second interface is connected to the first interface by the switcher and the maintenance port is connected to the control device by the switch, a maintenance state in which the maintenance port is connected to the first interface by the switcher and the maintenance port is connected to the control device by the switch, and a maintenance disable state in which the second interface is connected to the first interface by the switcher but the maintenance port is disconnected from the control device by the switch, wherein the input-output apparatus further comprises: an engineering mode for engineering of the process control system allowing the input-output apparatus, when the setting apparatus sends a request for connection to the control device via the maintenance port, to transit from the online state to the maintenance state for settings and/or adjustments on the field device and on the input-output apparatus and allowing the input-output apparatus to transit back to the online state when the settings and/or adjustments are completed; and an operating mode for performing process control of the process control system when the input-output apparatus is in the maintenance disable state. an engineering mode for engineering of the process control system allowing the input-output apparatus, when the setting apparatus sends a request for connection to the control device via the maintenance port, to transit from the online state to the maintenance state for settings and/or adjustments on the field device and on the input-output apparatus and allowing the input-output apparatus to transit back to the online state when the settings and/or adjustments are completed; and an operating mode for performing process control of the process control system when the input-output apparatus is in the maintenance disable state.