An internal combustion engine (E) comprises a breather chamber (40) for performing gas liquid separation of blow-by gas produced within an engine main body (21), being formed on the inside of a head cover (26) which is connected to a cylinder head (25)in such a way as to hold a gasket (48) between itself and the cylinder head (25). An outlet pipe (39) is provided in the head cover (26) so as to lead out the blow-by gas from the breather chamber (40), and a hose (41) leads the blow-by gas to an air cleaner (29) side having an end portion joined to the outlet pipe (39). The influence upon the surrounding environment is suppressed at a lower level by releasing a pressure from a predetermined part on purpose, even if an internal pressure within the engine main body (21) is increased by blow-by gas. A release device (43) is provided in the outlet pipe (39) in order for releasing blow-by gas outwardly by separating a hose (41) from the outlet pipe (39) when the pressure of the blow-by gas within an engine main body (21) is not less than a predetermined pressure.