A leakage detector for a fuel vapor processing system includes a pressure sensor and a control unit. The pressure sensor is configured to measure an internal pressure of the fuel vapor processing system. The control unit is configured to calculate changes of a fuel vapor pressure from a saturation vapor pressure curve of the fuel vapor, a fuel vapor concentration of the fuel vapor in a gaseous layer within the fuel vapor processing system, and a convective velocity in the gaseous layer, to correct a reference pressure for leakage detection based on the changes of the fuel vapor pressure so as to calculate a corrected reference pressure, and to compare the internal pressure measured by the first pressure sensor to the corrected reference pressure so as to determine whether any of the fuel vapor has leaked from the fuel vapor processing system.