A uniflow scavenging 2-cycle engine comprising: a cylinder in which a combustion chamber is formed; a piston configured to slide in the cylinder; scavenging ports formed in an inner circumferential surface of one end side of the cylinder in a stroke direction of the piston, and configured to suction an active gas into the combustion chamber according to sliding motion of the piston; fuel injection valves configured to inject a fuel gas into the active gas suctioned from the scavenging ports into the combustion chamber to generate a premixed gas; an exhaust port formed in the other end of the cylinder in the stroke direction of the piston; an exhaust valve configured to open and close the exhaust port; and an exhaust valve driving mechanism configured to open and close the exhaust valve such that a time required to displace the exhaust valve from a fully opened position to a fully closed position is longer than a time required to displace the exhaust valve from the fully closed position to the fully opened position, and characterized by comprising: a blow-by detector configured to detect an amount of a fuel gas component contained in the exhaust gas exhausted from the exhaust port as an amount of blow-by of the fuel gas from the exhaust port, and an exhaust controller configured to output an exhaust valve operation signal for controlling a degree of opening of the exhaust valve to the exhaust valve driving mechanism according to the detected amount of blow-by. a cylinder in which a combustion chamber is formed; a piston configured to slide in the cylinder; scavenging ports formed in an inner circumferential surface of one end side of the cylinder in a stroke direction of the piston, and configured to suction an active gas into the combustion chamber according to sliding motion of the piston; fuel injection valves configured to inject a fuel gas into the active gas suctioned from the scavenging ports into the combustion chamber to generate a premixed gas; an exhaust port formed in the other end of the cylinder in the stroke direction of the piston; an exhaust valve configured to open and close the exhaust port; and an exhaust valve driving mechanism configured to open and close the exhaust valve such that a time required to displace the exhaust valve from a fully opened position to a fully closed position is longer than a time required to displace the exhaust valve from the fully closed position to the fully opened position, and characterized by comprising: a blow-by detector configured to detect an amount of a fuel gas component contained in the exhaust gas exhausted from the exhaust port as an amount of blow-by of the fuel gas from the exhaust port, and an exhaust controller configured to output an exhaust valve operation signal for controlling a degree of opening of the exhaust valve to the exhaust valve driving mechanism according to the detected amount of blow-by. a blow-by detector configured to detect an amount of a fuel gas component contained in the exhaust gas exhausted from the exhaust port as an amount of blow-by of the fuel gas from the exhaust port, and an exhaust controller configured to output an exhaust valve operation signal for controlling a degree of opening of the exhaust valve to the exhaust valve driving mechanism according to the detected amount of blow-by.