A leak diagnosis method for a blowby gas treatment device of an internal combustion engine, wherein the blowby gas treatment device includes: a fresh air induction pipe structured to induce fresh air into a crank case from a section of an intake passage downstream of an air flow meter; a first blowby gas pipe structured to induce blowby gas from the crank case into a section of the intake passage upstream of a compressor; a second blowby gas pipe structured to induce blowby gas from the crank case into a section of the intake passage downstream of the compressor and downstream of a throttle valve; a shutoff valve structured to block the fresh air induction pipe; and a one-way valve structured to allow only a gas flow in the first blowby gas pipe from the crank case to the intake passage; the leak diagnosis method comprising: performing a first diagnosis to close the shutoff valve under a non-supercharging condition and determine whether falling of a pressure in the crank case after the closing of the shutoff valve is normal or abnormal; performing a second diagnosis to determine whether or not a sensed intake air quantity (Qafm) is equal to an actual intake air quantity (Qcylin) under the non-supercharging condition, wherein the sensed intake air quantity (Qafm) is a quantity of intake air sensed in the intake passage, and wherein the actual intake air quantity (Qcylin) is a quantity of intake air flowing into a cylinder set; determining presence of a leak to an outside of a system of the blowby gas treatment device, in response to a condition that the first diagnosis determines the falling of the pressure as abnormal and the second diagnosis determines the sensed intake air quantity (Qafm) as unequal to the actual intake air quantity(Qcylin); and determining presence of a failure within the system of the blowby gas treatment device, in response to a condition that the first diagnosis determines the falling of the pressure as abnormal and the second diagnosis determines the sensed intake air quantity (Qafm) as equal to the actual intake air quantity(Qcylin). performing a first diagnosis to close the shutoff valve under a non-supercharging condition and determine whether falling of a pressure in the crank case after the closing of the shutoff valve is normal or abnormal; performing a second diagnosis to determine whether or not a sensed intake air quantity (Qafm) is equal to an actual intake air quantity (Qcylin) under the non-supercharging condition, wherein the sensed intake air quantity (Qafm) is a quantity of intake air sensed in the intake passage, and wherein the actual intake air quantity (Qcylin) is a quantity of intake air flowing into a cylinder set; determining presence of a leak to an outside of a system of the blowby gas treatment device, in response to a condition that the first diagnosis determines the falling of the pressure as abnormal and the second diagnosis determines the sensed intake air quantity (Qafm) as unequal to the actual intake air quantity(Qcylin); and determining presence of a failure within the system of the blowby gas treatment device, in response to a condition that the first diagnosis determines the falling of the pressure as abnormal and the second diagnosis determines the sensed intake air quantity (Qafm) as equal to the actual intake air quantity(Qcylin).