To make it possible to monitor opening/closing of a manual valve if at least either the opening state or closing state of a manual valve can be detected.

A manual valve opening/closing state management device includes a detachable valve opening/closing state detector 12 for detecting the opening/closing state of the manual valve 11 to output an opening/closing state indicating signal, an attribute input device 13 for inputting an attribute signal for indicating whether the opening/closing state indicating signal is effective or not, a valve opening/closing management unit 15, a communication means 14 for transmitting the opening/closing state indicating signal and the attribute signal to the valve opening/closing management unit 15, a valve opening/closing state recorder 16, and a valve opening/closing state display unit 18. When the valve opening state indicating signal is effective and further when the valve opening state indicating signal indicates the opening state of the valve 11, the valve opening/closing management unit 15 judges that the manual valve 11 is in an open state, while when the valve closing state indicating signal is effective and further when the valve closed state indicating signal indicates the closed state of the valve 11, the valve opening/closing management unit 15 judges that the manual valve 11 is in a closed state. Further, when not judging that the valve is in the open or closed state, the valve opening/closing management unit 15 judges that the manual valve 11 is in an indefinite state to output a signal for indicating the state.