This article presents a resonance-type vibration energy harvester with a Duffing-type nonlinear oscillator that can perform effectively in a wide frequency range. To mitigate the power-bandwidth trade-off in conventional linear harvesters, the resonance frequency band of the harvester is expanded by introducing a Duffing-type nonlinear oscillator in order to enable the harvester to generate larger electric power in a wider frequency range. Such a nonlinear oscillator, however, can have multiple stable steady-state responses in the resonance band with different levels of regeneration energy. In this study, the principle of self-excitation is utilized to destabilize the solutions, except for the highest energy solution. A load circuit with a switch between the conventional load circuit and a negative resistance circuit and the switching control law, which depends on the amplitude of the oscillator’s response, are introduced to impart the self-excitation capability in order to entrain the oscillator with the excitation only in the highest energy solution. Theoretical and numerical analyses are conducted to show that the proposed harvester can respond in a large amplitude in a wide frequency range, and a significant improvement in the regenerated power is achieved as compared to the one without self-excitation control.