The invention concerns a safety device for a window unit (10) with a mobile shutter (18) comprising a glass panel (16) connected over at least a portion of its periphery to a structure (12) bearing said window unit, said panel including an aperture (22) for said mobile shutter, said shutter (18) being made mobile by at least one of its peripheral edges by a linking member (28) co-operating with a section (20) integral with the panel (16) and forming a housing for at least part of the linking member. The invention is characterised in that the safety device comprises, at each end of the section (20) a safety subassembly (100) comprising at least a longitudinal safety element and linking means extending between said safety element and said structure (12) such that, when said panel (16) is broken, said section (20), and optionally, said shutter (18) are retained on said structure (12) by said safety elements and said linking means. The invention is applicable to window units with tilting or sliding of vehicles.