A connection device for fluid lines has a housing in which at least one housing recess is formed stepped in the longitudinal direction and merging at an inner end thereof with a fluid duct. An attachment means is fixed in the housing recess and defines a passage opening rendering possible the plugging into position of a fluid line to be connected. A sealing ring is associated with said attachment means. The sealing ring has an inner periphery matching the outline of the fluid line to be connected and being penetrated by the fluid line when connected. A centering section of the housing recess is located axially within the attachment means. An adapter ring is provided able to be inserted into the centering section for bridging over any radial intermediate space present between the outer periphery of the line's end section and the inner periphery of the centering section.