The invention provides an abnormity diagnosis device for a blow-by gas recovery device in an engine with a supercharger. A leakage abnormity of the blow-by recovery device can be diagnosed in high precision through a simple structure. The device can be used for coping with engine deceleration stoppage caused by sucking the residual EGR gas. A BGV device comprises a gas storage part, a throttle, aBGV passage, a PCV valve and a fresh gas introducing passage which introduces fresh gas into the gas storage part. The abnormity diagnosis device comprises an air flowmeter which detects an air incoming amount at the air inlet passage which is at the upstream part of the throttle, a fresh air branch passage which is branched from the fresh air introducing passage and introduces fresh air to the air inlet passage at the downstream part of the throttle, a fresh air introducing valve of the fresh air branch passage, and an electronic control device for diagnosis. When the deceleration fuel of theengine is off, an ECU closes and opens the fresh air introducing valve, and the leakage abnormity of the BGV device according to a closed-valve detection air inlet amount and an open-valve detectionair inlet amount which are detected by the air flowmeter.