Previous studies by the authors show that energy regenerative vibration control systems can be realized by using piezoelectric elements as actuators and class D amplifiers as drivers of the piezoelectric elements. This paper shows a novel control method suitable for the energy regenerative vibration control systems. The control method is a kind of modal control and adapts modal feedback gains to maximize control performance under the following two constraints. One constraint is the condition for the balance of the regenerated energy and the energy losses in the systems. The condition is satisfied passively by using capacitors as the power sources of the amplifiers. The other is the input constraint of the class D amplifiers and satisfied by using gain adaptation mechanisms of controllers. The mechanisms need an index of the vibration amplitude of each mode and the modal energy is used as the index. The validity of the proposed method is shown by numerical simulations of a cantilever beam with piezoelectric elements.