1. A control device comprising: a communication unit which has a switch function of switching the control device into a power saving mode, the communication unit communicating with an upper terminal via a first network; a mode control unit which controls switching the communication unit into the power saving mode, the control being made using a network control library which is generated by a programming language; a memory unit which stores a monitoring time and a frame classification as a mode control parameter, the monitoring time being a time for monitoring a transmission-reception frame transmitted and received by the communication unit, the frame classification designating a group of frames which are subject to the monitoring; and a transmission-reception frame information management unit which manages a frame-related information that includes the monitoring time and the frame classification, wherein the network control library is a software component which has a function of outputting a value which is necessary to control switching the control device into the power saving mode, the control being made according to at least one input value, the mode control unit comprises: a remote instruction detection unit which detects the input value which is included in a remote instruction given by a remote monitoring terminal via a second network which is different from the first network, and the remote instruction detection unit outputting an output value as a mode control signal, the output value being obtained from the network control library based on the detected input value; and a power saving control unit which controls switching the control device into the power saving mode, the control being made based on the mode control signal, the power saving unit determines whether or not in the monitoring time there is performed at least one of transmission and reception of the transmission-reception frame classified in the group of frames subject to the monitoring, the determination being made based on the frame-related information, and the power saving unit selects the power saving mode if in the monitoring time there is not performed the at least one of transmission and reception of the transmission-reception frame.