A leak detecting system for a transpiration purge system including a fuel tank 1 and a canister 13 includes a jet pump 8, which leads the atmosphere into the fuel tank 1 using a return flow of excess fuel of fuel pumped to an internal combustion engine, internal pressure measuring means 15, and fuel volume detecting means 18. After supplying the air from pressurizing means for a predetermined time, the leak detecting system judges that a leak is present when a holding pressure measurement value in a state in which the transpiration purge system is closed is lower than a decompression calculation value calculated from a pressurization measurement value, an air volume, and a leak hole diameter.