A system of an autonomous intracardiac capsule and its implantation accessory. The autonomous capsule (10) includes a tubular body (12) with an anchoring screw (14) for penetrating the wall of a cavity of the heart, and at least one coupling finger (20, 22) radially projecting outwards. The implantation accessory (26) includes a lead body (28) with a sheath (30, 32) of deformable material supporting on the distal side a helical guide (36, 52), for guiding and driving by rotation the capsule. This helical guide is integral with the lead body, and its inner diameter is homologous to the outer diameter of the cylindrical body of the capsule so the latter can be housed it in, the coupling fingers protruding between the coils of the guide. The helix direction of the helical guide (36) is opposite to that of the anchoring screw (14). The helical guide is resiliently compressible in axial direction, and its helix pitch (38) is increased in the free distal end portion (40).