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 pressure control valve disposed in a section of the intake passage upstream of a throttle valve and upstream of a compressor; a first blowby gas pipe structured to induce blowby gas from the crank case into a section of the intake passage between the compressor and the pressure control valve; 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 the throttle valve; a shutoff valve structured to block the fresh air induction pipe; 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; and a positive crankcase ventilation valve disposed in the second blowby gas pipe; the leak diagnosis method comprising: performing a first diagnosis to close the shutoff valve under a 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 control the pressure control valve into an open-side state and a closed-side state under a non-supercharging condition and determine whether or not an open-side state intake air quantity (Qopen) is equal to a closed-side state intake air quantity (Qclose), wherein the open-side state intake air quantity (Qopen) is a quantity of intake air measured when the pressure control valve is in the open-side state, and wherein the closed-side state intake air quantity (Qclose) is a quantity of intake air measured when the pressure control valve is in the closed-side state; 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 open-side state intake air quantity (Qopen) as unequal to the closed-side state intake air quantity (Qclose); 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 open-side state intake air quantity (Qopen) as equal to the closed-side state intake air quantity (Qclose). performing a first diagnosis to close the shutoff valve under a 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 control the pressure control valve into an open-side state and a closed-side state under a non-supercharging condition and determine whether or not an open-side state intake air quantity (Qopen) is equal to a closed-side state intake air quantity (Qclose), wherein the open-side state intake air quantity (Qopen) is a quantity of intake air measured when the pressure control valve is in the open-side state, and wherein the closed-side state intake air quantity (Qclose) is a quantity of intake air measured when the pressure control valve is in the closed-side state; 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 open-side state intake air quantity (Qopen) as unequal to the closed-side state intake air quantity (Qclose); 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 open-side state intake air quantity (Qopen) as equal to the closed-side state intake air quantity (Qclose).