Abstract: A plant control device includes: a terminal base where a wiring is connected which is connected to each of multiple devices installed in a plant; a control IO module where an IO signal is input or the IO signal is output; and a terminal base adapter disposed between the control IO module and the terminal base, the terminal base adapter being configured to convert, by a predetermined logic, an IO signal input from at least one of the multiple devices or the IO signal input from the control IO module. The terminal base adapter is mountable on the terminal base and includes a second electrical connector which is mounted with a first electrical connector disposed on the control IO module.