A connection device for connecting a main pipe (1) to a secondary pipe (2). The connection device includes a substantially cylindrical connection piece (3), able to be inserted, preferably able to be screwed, into a wall opening of the main pipe (1), for connection of the secondary pipe (2), wherein for sealing the connection piece (3) a preferably ring-shaped sealing element (4) is provided, lying against the main pipe (1), which sealing element is connected directly or via mechanical connecting elements to an outer circumference of the connection piece (3).