The invention relates to an assistive device for positioning a medical instrument (2) inserted into a natural duct (100) or an artificial duct of a patient relative to an internal organ (P) of a patient, wherein the device comprises a support (1) that is to be inserted at least in part into the body of the patient and supports the medical instrument, means for moving the support, wherein said movement means comprise an articulated arm (9) that includes a plurality of degrees of freedom for moving a proximal end (1a) of the support, means for acquiring images of the internal organ for positioning the medical instrument relative to the internal organ, wherein the image acquisition means comprise a probe (5) supported by the support such that the medical instrument and the probe are rigidly connected, and a control unit (10) for controlling the movement means, which is connected to the image acquisition means and comprises image analyzing means (11) for generating control commands for the articulated arm in order to control at least one movement of the medical instrument relative to the internal organ. The invention further relates to a method for controlling such a device.