A guide device (1) for introducing at least a surgical instrument internally of an organic cavity (2), in particular a vascular cavity comprises a guide body or guide catheter (3) which is substantially hollow and flexible and exhibits an open directing end (8) which is transversal. The device comprises an elongate and flexible guide element (9), which is slidable internally of the guide body (3) to define an advancing guide for the guide body (3). The device (1) also includes means for stabilising (10) for easily piloting the directing end (8) with respect to the transversal cavity (2b) and for advancing the guide catheter. The means for stabilising are defined by a stabilising opening (11) and by an elongate and flexible stabilising element (12) which internally engages the guide body (3) and exits therefrom through the stabilising opening (11) along the cavity occupied by the guide body (3). The stabilising element (12) behaves as a rotation pivot by means of which the operator doing the operation can easily align the directing end (8) of the guide body (3) with the transversal cavity (2b), and guide the directing end (8) into the transversal cavity (2b).