A leakage detector for a fuel vapor treatment device includes a fuel vapor treatment device that maintains the interior of a treatment system of the fuel vapor treatment device in a sealed state. In addition, the leakage detector includes; a closing valve and shutoff valve that can shut off fluid communication between the fuel tank and the canister to section the interior of the treatment system into a first region including the fuel tank and a second region including the canister. Further, the leakage detector includes an aspirator that applies a negative pressure to the second region. The leakage detector for the fuel vapor treatment device determines the presence and/or absence of leakage in the first region on the basis of a change in the pressure in the first region while an engine is stopped and determines the presence and/or absence of leakage in the second region on the basis of a change in the pressure in the second region by applying a negative pressure to the second region while the engine is operated.