The pneumatic actuator has high potential as an actuator for mobile devices due to its high power density, simplicity, compactness, light weight, and low cost. Moreover, inherent compliance of the pneumatic actuator improves safety and dynamic performance, which are strongly required for wearable robots. However, due to its need for a bulky air compressor, which is intended for stationary use, it is hard to apply a pneumatic actuator to mobile robots. To replace the air compressor, pneumatic energy sources utilizing chemical fuel were previously proposed and the potential of the hydrogen peroxide as a pneumatic energy source was proved. Nevertheless, pneumatic energy sources driven by hydrogen peroxide are generally bulky, which are hard to apply for a mesoscale system such as prosthetics. To address this challenge, this paper proposes novel pneumatic power generator driven by hydrogen peroxide. By applying a passive injection component and a mechanical pressure feedback mechanism, the pneumatic power generator regulates pressure by itself, without any control efforts, which results in compact size. In addition, a separated catalytic reactor and a fuel tank guarantee safety and mobility. The performances and energetic characteristics of the proposed pneumatic power generator are examined by both experiment and mathematical modeling.