This paper presents a study of tracking accuracy improvement of marker-free bronchoscope tracking using an electromagnetic tracking system. Bronchoscope tracking is an important function in a bronchoscope navigation system that assists a physician during bronchoscopic examination. Several research groups have presented a method for bronchoscope tracking using an ultra-tiny electromagnetic tracker (UEMT) that can be inserted into the working channel of a bronchoscope. In such a system, it is necessary to find the matrix T showing the relation between the coordinate systems of the CT image and the UEMT. This paper tries to improve the accuracy of this matrix by using not only the position information of the UEMT but also the orientation information. The proposed algorithm uses the running direction information of bronchial branches and the orientation information of the UEMT in the computation process of T. In the experiments using a bronchial phantom, the tracking accuracy was improved from 2.2 mm to 1.8 mm.