A connection device may include an insertion section having an annular outer groove. A housing may be configured to receive the insertion section. A blocking element may be configured to lock the insertion section in the housing, wherein the blocking element may engage the annular outer groove and an annular inner groove formed in the housing. The blocking element may also be configured to be withdrawn through a housing opening substantially tangentially to the grooves and to be pre-fitted in the inner groove and in a pre-fitted state. The blocking element may engage the outer groove and be fixed on the housing securing it against a displacement in a peripheral direction. A securing device may be arranged on an exterior of the housing for securing of the blocking element against displacement at least in the withdrawal direction.