A leakage detector for a fuel vapor treatment device includes a control unit. The control unit shifts a purge control valve and a shutoff valve to a first state with the purge control valve is an opened position and the shutoff valve in a closed position, and then shifts the purge control valve and the shutoff valve to a second state with the purge control valve is a closed position and the shutoff valve in a closed position. The control unit compares a first purge passage pressure acquired from the purge passage pressure detector when the purge control valve and the shutoff valve were shifted to the second state with a second purge passage pressure acquired from the purge passage pressure detector after a lapse of a first waiting time after being shifting to the second state. This comparison is used to determine leakage in the purge passage.