For fixation of a trocar comprising a shaft (1) and an external sleeve (3) the trocar is provided with a spreading device which prevents unintentional withdrawal of an external sleeve (3) of the trocar. The spreading device comprises shell-shaped tongues (7) formed from parts of the external sleeve (3). By a rotating movement of an actuation element (9) the tongues (7) can be folded transversely to their longitudinal axis in such a way that they project radially beyond the external sleeve (3). <IMAGE>
