We have found the optimal conditions for work performed by the quantum Szilard engine (SZE) containing multi-particles under isothermal processes with inevitable irreversibility. We define the restorability as the probability that the reversibility of an engine with inevitable irreversibility is achieved when the time-backward process is performed. The optimal condition is then obtained when the restorability is maximized. This is equivalent to the condition that the time-forward force is equal to the time-backward force. We numerically confirm our expectation by using the quantum SZE containing three bosons or fermions.