An engine-driven breaker which is capable of providing a breaking force equal to that of a hydraulic or pneumatic breaker even with a small engine and which is small and light and easy to handle and has a simple construction enabling it to be manufactured cheaply. The engine-driven breaker has a breaker main body and a cylinder fixed inside the breaker main body and inside the cylinder an upper air chamber of variable volume is provided between an operating piston and a hammer piston and a lower air chamber of variable volume is provided below the hammer piston. An annular air passage connected by way of a throttling passage to a crank chamber housing a crank mechanism driving the operating piston is provided between the outer peripheral wall of the cylinder and the inside wall of the breaker main body, and first air holes and second air holes are provided in the cylinder wall passing through to the annular air passage in a position corresponding to a bottom dead center position of the hammer piston and in a position below that respectively. Upper air holes for connecting the upper air chamber and the crank chamber are also provided in the cylinder wall in the crank chamber just below a top dead center position of the operating piston.