A learned neural network learned in weights using an engine load, an engine speed, and an intake pressure inside the engine intake passage downstream of the throttle valve (19) as input parameters of the neural network and using leakage of blow-by gas from a blow-by gas feed path (20) as a truth label is stored. At the time of operation of the vehicle, the learned neural network is used to detect the abnormality of leakage of blow-by gas from the blow-by gas feed path (20) from the above input parameters.