An engine brake unit may include a rocker arm having a first space defined in a valve-pressing portion thereof, wherein the first space communicates with a supply passage, through which engine brake oil is introduced, a stopper fixedly provided above the first space of the rocker arm in a predetermined distance, an actuator located inside the first space and having a pressure piston, wherein the pressure piston is selectively raised by hydraulic pressure of the engine brake oil to butt against the stopper, thereby pressing a portion of the rocker arm downwards.