The objective of the invention is to make the use of a cannula belonging to a bioimpedance sensor easier in a medical procedure after defining the bioimpedance. The bioimpedance sensor (300) comprises a cannula (200) and a stylet (100) moveable in relation to it and is characterized in that:the cannula (200) comprises a needle tube (1), consisting of or containing electrically conductive material so that the cannula (200) is available for use as a needle electrode (1, 2) or as a part of it;the stylet (100) has a bevelled head and comprises a number of stylet electrodes (5) surrounded by electrical insulation (13) in such a manner that the beveled head is left free from electrical insulation so that the advance of the needle tip can be characterized by measuring the impedance between the needle electrode (1,2) and the number of stylet electrodes (5);the stylet (100) is additionally equipped with at least one coupling piece (6) for the electrical coupling of the stylet (100) to the needle electrode (1, 2) inside the cannula (200), whereby the impedance between the needle electrode (1, 2) and at least one of the stylet electrodes (5) can be measured inside the bioimpedance sensor (300) and exclusively through the stylet (100).The patent application contains independent claims also for the stylet, the cannula and the method for measuring bioimpedance.