It is difficult to satisfy the collocation requirement in the vibration control system of a cantilever beam with a piezoelectric actuator. Since not all modes of the cantilever beam can be stabilized with direct velocity feedback control, state-space methods must be used for the mechanical system. Though state-space methods are very useful, many sensors whose number is at least equal to the number of multiple eigenvalues concerned are needed to fulfill observability conditions. In this paper, we describe a control method with which eigenvalues whose number exceeds that of sensors are controlled using digital filters and inverse systems based on truncated mechanical models. From experimental results, it is found that five natural frequencies of the cantilever beam are almost independently damped with the above control method using one piezoelectric actuator and three strain gages.