A leak detecting apparatus includes a canister adsorbing a fuel vapor evaporated in a fuel tank, a measure passage, a pump connected with the canister through the measure passage, and a pressure senor detecting a pressure in the measure passage. The pump depressurizes the measure passage, the canister, and the fuel tank so that a leakage of the fuel vapor is detected. When a blow-by of the fuel vapor is arisen, the pump is stopped to forcibly terminate a depressurization.