A pressure sensor is provided in a purge pipe connecting a fuel tank to a canister, so that the pressure sensor outputs an electrical signal of a first sensor value depending on an inside pressure of the canister. A control unit calculates an estimated sealed pressure, which corresponds to the inside pressure of the canister in a condition that the canister and the fuel tank are in a sealed condition. The control unit determines that there is no leakage of fuel vapor, when the first sensor value is out of a first pressure range. The control unit determines that there is a small leakage of the fuel vapor, when the first sensor value is within the first pressure range but out of a second pressure range, wherein the second pressure range is smaller than the first pressure range. The control unit determines that there is a large leakage or a blocked-up condition in the purge pipe when the first sensor value is within the second pressure range.