To provide a leak detection device capable of inspecting the presence of a minute clearance by use of an inert gas in an easy method.

This device comprises a work sealing means 70 (10, 30) for sealing a work 150 from the upper and lower parts thereof, an inert gas supplying means 80 for supplying the inert gas into a cavity 33 provided on one side of the work 150 within the work sealing means 70, and an inert gas detector 110 connected to a cavity 11 provided on the other side of the work 150 within the work sealing means 10 to evacuate the gas in the cavity 11 and detect the presence of the inert gas. The inert gas supplied from the inert gas supplying means 80 to the cavity 33 is detected by the inert gas detector 110 connected to the cavity 11, whereby the leak of the work is inspected.