A total amount of engine oil carried from a crankcase into an intake passage by blow-by gas from the start of the use of the internal combustion engine or the engine oil is calculated. It is determined whether the total oil carried amount is greater or less than an optimum value every predetermined operation period of the internal combustion engine. Then, when it is determined that the total oil carried amount is greater than the optimum value, the gas flow rate of the blow-by gas controlled based on the engine operation state is reduced until next determination process is performed. When it is determined that the total oil carried amount is less than the optimum value, the gas flow rate is increased until next determination process is performed.