Injection device for an internal combustion engine comprises an engine cylinder head part having a recess accessible by an insertion opening defined by a base wall (15) and defining a seal seat (11), an injection valve (2) with a tubular body (7) arranged inside the recess and protruding from the recess partly inside the insertion opening and defining a radial intermediate space (19) with a side surface (18) of the insertion opening, and a sealing device (5) supported by the injection valve. The sealing device comprises a bellows-shaped sealing organ (21) sliding onto the tubular body of the injection valve and having a sealing lip (28) lying axially on the seal seat. Preferred Features : The seal seat is funnel-shaped and axially tapers in the direction of the recess.