The leak detection device is adapted to test the tightness of an inner tank (24) of a valve cell (20) of the vacuum deposition apparatus, either at its filling flange (25) or at its inner tank valve (28). A vacuum deposition apparatus equipped with a helium detector (51) mounted as a by-pass of the output of a high-flow-rate turbomolecular pump (42) which is connected to the vacuum deposition chamber (30) of the vacuum deposition apparatus by a slide gate valve (43). A valve-cell vacuum deposition apparatus equipped with a helium-based leak detection device including gas injection elements (52, 53) adapted to inject a gaseous mixture into the outer enclosure, the gaseous mixture being consisted of pure helium and an inert gas, and a method for detecting a leak in a valve-cell vacuum deposition apparatus are also described.