A vibration-driven micropower generator with a high-Q mechanical resonator efficiently works under single-frequency vibration. However, the associated narrow operation frequency bandwidth tightly limits the power output under commonly observed wideband ambient vibrations. In this paper, we present a power-generation scheme in which the motion of an internal mass element is actively controlled. The control is such that the vibrating environment is forced to do the maximum possible work to the power generator. Electret-based energy conversion was analyzed as a means to realize the scheme, drawing on evidence from an experiment. We further analyzed the active control in general terms to reveal potential performance that is substantially above the conventional designs.