A device for preventing dilution of engine oil includes an oil pan in which oil for lubrication of the engine is recovered with fuel. An exhaust purifier is provided on a rear end of the engine and purifying exhaust gas. A gas passage can guide flow of exhaust gas passing through the exhaust purifier to the oil pan. A gas valve can control flow rate of the exhaust gas flowing in the gas passage. A controller controls an opening ratio of the gas valve depending on a temperature of oil inside the oil pan to maintain the temperature of the oil equal to or higher than a temperature at which the fuel in the oil pan is vaporized.