Connecting device having a plug-in connector and a mating plug-in connector, wherein the plug-in connector has a housing, which has a plug-in side, a rear side and a base side that connects the plug-in side and the rear side, and a latching arm, the mating plug-in connector has a receiving space, in which the plug-in connector can be introduced at least partially in an insertion direction into a connecting position, wherein, in the connecting position, the latching arm, in a rest position, is in engagement with a latching receptacle of the mating plug-in connector, the plug-in connector has a securing unit, which is arranged on the base side so as to be able to move between a locking position, in which the securing unit locks the latching arm in the rest position, and a released position, in which the latching arm can be moved out of its rest position, the securing unit has a base part and a securing arm that extends away from the base part in the direction of the plug-in side, the securing arm has an abutment surface, which, in the released position, abuts against a stopper element arranged on the base side of the housing, so that the securing unit is blocked from moving into the locking position, the securing arm has, at an end facing away from the base part, a deflecting slope, the mating plug-in connector has at least one mating abutment element, which, when the plug-in connector is introduced into the receiving space, abuts against the deflecting slope and deflects the securing arm, so that, in the connecting position, the abutment surface is arranged so as to be offset in relation to the stopper element and the securing unit can be moved into the locking position, characterized in that the securing arm is deflected perpendicularly to an insertion direction and parallel to the base side, the base side is divided into a first and a second base side portion (20; 21) by a median plane, which extends centrally from the rear side to the plug-in side and perpendicularly to the base side, and the stopper element and at least a large proportion of the securing arm are arranged in the second base side portion. a latching arm, the mating plug-in connector has a receiving space, in which the plug-in connector can be introduced at least partially in an insertion direction into a connecting position, wherein, in the connecting position, the latching arm, in a rest position, is in engagement with a latching receptacle of the mating plug-in connector, the plug-in connector has a securing unit, which is arranged on the base side so as to be able to move between a locking position, in which the securing unit locks the latching arm in the rest position, and a released position, in which the latching arm can be moved out of its rest position, the securing unit has a base part and a securing arm that extends away from the base part in the direction of the plug-in side, the securing arm has an abutment surface, which, in the released position, abuts against a stopper element arranged on the base side of the housing, so that the securing unit is blocked from moving into the locking position, the securing arm has, at an end facing away from the base part, a deflecting slope, the mating plug-in connector has at least one mating abutment element, which, when the plug-in connector is introduced into the receiving space, abuts against the deflecting slope and deflects the securing arm, so that, in the connecting position, the abutment surface is arranged so as to be offset in relation to the stopper element and the securing unit can be moved into the locking position, characterized in that the securing arm is deflected perpendicularly to an insertion direction and parallel to the base side, the base side is divided into a first and a second base side portion (20; 21) by a median plane, which extends centrally from the rear side to the plug-in side and perpendicularly to the base side, and the stopper element and at least a large proportion of the securing arm are arranged in the second base side portion.