Disclosed is a leak detecting device for a fuel vapor treatment unit that purges a vapor gas generated through evaporation of fuel in a fuel tank (2) into an intake system (22) of an engine (21). The leak detecting device includes a valve (5) that can selectively seal the fuel vapor treatment unit; a pressure detecting sensor (8) that detects a pressure in the fuel vapor treatment unit; and a controller (10). The controller (10) is programmed to: issue a command to close the valve (5) with a view to sealing the fuel vapor treatment unit during stoppage of the engine (21); calculate deviation amounts (P-P.sub.0) of the detected pressures during stoppage of the engine (21) after the fuel vapor treatment unit has been sealed; integrate absolute values (|P-P.sub.0|) of the deviation amounts, and determine, based on an integrated value (s), whether or not there is a leak occurring in the fuel vapor treatment unit.