LEAK DIAGNOSIS METHOD AND LEAK DIAGNOSIS DEVICE FOR BLOW-BY GAS TREATMENT DEVICE FOR INTERNAL COMBUSTION ENGINE

A blowby gas treatment device includes a pressure control valve (18), a fresh air induction pipe (21), a first blowby gas pipe (22), a second blowby gas pipe (23), a shutoff valve (24), a one-way valve (25), and a PCV valve (26). A leak diagnosis (A) includes a first-stage diagnosis to determine whether or not falling of a pressure in a crank case (3) after closing of the shutoff valve (24) under a non-supercharging condition is normal, and a second-stage diagnosis performed when the falling of the pressure is determined as abnormal by the first-stage diagnosis. The second-stage diagnosis is implemented by using an air fuel ratio feedback control, and determining whether or not a sensed intake air quantity obtained by an air flow meter (19) is equal to an actual intake air quantity flowing into a cylinder set. When the sensed intake air quantity is determined as equal to the actual intake air quantity, presence of leaking within a system is determined, and when the actual intake air quantity is determined as larger, presence of leaking to an outside of the system is determined.