  When the process of a system in contact with a heat bath is described by
classical Langevin equation, the method of stochastic energetics [K. Sekimoto,
J. Phys. Soc. Jpn. vol. 66 (1997) p.1234] enables to derive the form of
Helmholtz free energy and the dissipation function of the system. We prove that
the irreversible heat Q_irr and the time lapse $Delta t} of an isothermal
process obey the complementarity relation, Q_irr {Delta t} >= k_B T S_min,
where S_min depends on the initial and the final values of the control
parameters, but it does not depend on the pathway between these values.
