An engine brake unit, may include a rocker shaft having an oil passage to open or close an exhaust valve when an engine braking is in operation, an exhaust rocker arm rotatable about the rocker shaft inserted into the exhaust rocker arm, wherein the exhaust rocker arm includes a supply oil passage communicating with the oil passage and the outside, and a recess connected to the supply oil passage and having an open lower portion, an actuator disposed in the recess of the exhaust rocker arm, wherein the actuator includes a piston that selectively moves downwards through the open lower portion of the recess by oil supplied from the supply oil passage to press the exhaust rocker arm while oil pressure in the oil passage has a predetermined pressure or more, and an oil control valve connected to the oil passage of the rocker shaft and controlling the oil pressure.