An apparatus (100, 200) for purging a crank case of a duel fuel engine, the apparatus comprising: a purge gas supply unit (110, 210) configured to supply purge gas for purging an inside of the crank case; a supply line (120, 220) configured to provide a path for supplying the purge gas to the inside of the crank case from the purge gas supply unit; an open/close unit installed in the supply line (120, 220) and configured to open and close supply of the purge gas; an exhaust line (140, 240) installed in the crank case for exhaustion; and a detection unit (150, 250) configured to detect fuel gas present inside the crank case wherein the detection unit (150, 250) comprises: a sampling tube (151, 251) connected to the crank case and comprising a cover installed on an end thereof placed inside the crank case to prevent lubricating oil from being attached; a pumping unit configured to extract gas from the crank case through the sampling tube; a gas sensor (153, 253) installed in the sampling tube and configured to detect fuel gas contained in the gas extracted from the crank case; a flow meter (161, 261) installed in the sampling tube and configured to detect a quantity of the gas extracted from the crank case; and a flame arrest (154, 254) installed in the sampling tube (151, 251) before the gas sensor (153, 253) and configured to block flames from being transferred to the crank case. a purge gas supply unit (110, 210) configured to supply purge gas for purging an inside of the crank case; a supply line (120, 220) configured to provide a path for supplying the purge gas to the inside of the crank case from the purge gas supply unit; an open/close unit installed in the supply line (120, 220) and configured to open and close supply of the purge gas; an exhaust line (140, 240) installed in the crank case for exhaustion; and a detection unit (150, 250) configured to detect fuel gas present inside the crank case wherein the detection unit (150, 250) comprises: a sampling tube (151, 251) connected to the crank case and comprising a cover installed on an end thereof placed inside the crank case to prevent lubricating oil from being attached; a pumping unit configured to extract gas from the crank case through the sampling tube; a gas sensor (153, 253) installed in the sampling tube and configured to detect fuel gas contained in the gas extracted from the crank case; a flow meter (161, 261) installed in the sampling tube and configured to detect a quantity of the gas extracted from the crank case; and a flame arrest (154, 254) installed in the sampling tube (151, 251) before the gas sensor (153, 253) and configured to block flames from being transferred to the crank case. a sampling tube (151, 251) connected to the crank case and comprising a cover installed on an end thereof placed inside the crank case to prevent lubricating oil from being attached; a pumping unit configured to extract gas from the crank case through the sampling tube; a gas sensor (153, 253) installed in the sampling tube and configured to detect fuel gas contained in the gas extracted from the crank case; a flow meter (161, 261) installed in the sampling tube and configured to detect a quantity of the gas extracted from the crank case; and a flame arrest (154, 254) installed in the sampling tube (151, 251) before the gas sensor (153, 253) and configured to block flames from being transferred to the crank case.