of Maxwell’s demon is a single-particle heat engine proposed by L. Szilard in 1929 [@Szilard]. During the thermodynamic cycle of the Szilard engine, the demon obtains $1$ bit ($= \ln 2$ nat) of information by a measurement, performs feedback control, and extracts $k_{\rm B}T \ln 2$ of positive work from a single heat bath. After numerous arguments on the consistency between the demon and the second law of thermodynamics, it is now understood that the work needed for the demon (or equivalently the feedback controller) during the measurement and information erasure compensates for the work that can be extracted by the demon [@Sagawa-Ueda2]. Therefore, we cannot extract a net positive work from the total system of the engine and the demon in an isothermal cycle, and therefore the presence of the demon does not contradict the second law of thermodynamics. Nevertheless, $k_{\rm B}T \ln 2$ of work extracted by the demon can be still useful. By using feedback control, we can increase the system’s free energy without injecting any energy (work) to it. We stress that, without feedback control, we need the direct energy input into the system in order to increase its free energy due to the second law 