An intracardiac capsule unit with its accessory for implantation in situ, comprising: - an autonomous capsule, comprising a cylindrical tubular body provided at its distal end with an anchoring member adapted to penetrate a tissue of a heart cavity wall; and - an accessory for implantation of the capsule, comprising:. a catheter with an internal lumen, extended at its distal end by a tubular protection tip defining an internal volume adapted to receive the capsule, and. decouplable means for supporting and guiding the capsule up to an implantation site, characterized in that: - the catheter is a tele-orientable catheter, and - the implantation accessory further comprises:. a sub-catheter received inside the lumen of the tele-orientable catheter with a relative translational degree of freedom and a relative rotational degree of freedom with respect to the tele-orientable catheter, the sub-catheter and the capsule being telescopically extendable with respect to the tele-orientable catheter between i) a retracted position where the capsule and its anchoring member are fully received inside the tubular protection tip, and ii) an extended position where the capsule is out of the tubular protection tip and is carried by the distal end of the sub-catheter, and - the distal end of the sub-catheter and the proximal region of the capsule are provided with first means (20, 84) for mutual translational and rotational fastening, these first fastening means being decouplable under the effect of a rotation applied to the sub-catheter from the proximal end thereof. - an autonomous capsule, comprising a cylindrical tubular body provided at its distal end with an anchoring member adapted to penetrate a tissue of a heart cavity wall; and - an accessory for implantation of the capsule, comprising:. a catheter with an internal lumen, extended at its distal end by a tubular protection tip defining an internal volume adapted to receive the capsule, and. decouplable means for supporting and guiding the capsule up to an implantation site,. a catheter with an internal lumen, extended at its distal end by a tubular protection tip defining an internal volume adapted to receive the capsule, and. decouplable means for supporting and guiding the capsule up to an implantation site, - the catheter is a tele-orientable catheter, and - the implantation accessory further comprises:. a sub-catheter received inside the lumen of the tele-orientable catheter with a relative translational degree of freedom and a relative rotational degree of freedom with respect to the tele-orientable catheter, the sub-catheter and the capsule being telescopically extendable with respect to the tele-orientable catheter between i) a retracted position where the capsule and its anchoring member are fully received inside the tubular protection tip, and ii) an extended position where the capsule is out of the tubular protection tip and is carried by the distal end of the sub-catheter, and. a sub-catheter received inside the lumen of the tele-orientable catheter with a relative translational degree of freedom and a relative rotational degree of freedom with respect to the tele-orientable catheter, - the distal end of the sub-catheter and the proximal region of the capsule are provided with first means (20, 84) for mutual translational and rotational fastening, these first fastening means being decouplable under the effect of a rotation applied to the sub-catheter from the proximal end thereof.