PURPOSE: To increase an engine brake force, by installing an air sealing chamber, interconnecting the inside of a combustion chamber, in a cylinder head and making a valve at the opening part open in time of operating an exhaust retarder.

CONSTITUTION: An air sealing chamber 7 interconnecting a combustion chamber 4 is installed in a cylinder head 3 having an exhaust valve 6 and a valve 8 is installed in the opening part as well. A throttle part 11 is installed in the air sealing chamber 7. The valve 8 is usually closed up by dint of a spring 9 but, in time of operating an exhaust retarder, is opened for operation by means of a solenoid 10. With this, both the combustion chamber 4 and the air sealing chamber 7 come into a state of being interconnected with each other whereby air inside the combustion chamber 4 flows in and out the air sealing chamber 7 together with up and down motions of a piston, producing pumping resistance there due to an action of the throttle part 11 and performing an engine braking action, so that braking force in time of operating the exhaust retarder is increased.