  We discuss thermalization of isolated quantum systems by using the
Husimi-Wehrl entropy evaluated in the semiclassical treatment. The Husimi-Wehrl
entropy is the Wehrl entropy obtained by using the Husimi function for the
phase space distribution. The time evolution of the Husimi function is given by
smearing the Wigner function, whose time evolution is obtained in the
semiclassical approximation. We show the efficiency and usefullness of this
semiclassical treatment in describing entropy production of a couple of quantum
mechanical systems, whose classical counter systems are known to be chaotic. We
propose two methods to evaluate the time evolution of the Husimi-Wehrl entropy,
the test-particle method and the two-step Monte-Carlo method. We demonstrate
the characteristics of the two methods by numerical calculations, and show that
the simultaneous application of the two methods ensures the reliability of the
results of the Husimi-Wehrl entropy at a given time.
