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.