We present the first measurement of a tokamak plasma internal magnetic field based on the analysis of polarization of Zeeman components of a magnetic dipole transition in heavy impurity ion. The circular polarization of the Ti XVII 3834 A˚ line is studied. The values of the component of the toroidal magnetic field in the direction of observation are measured for several observation angles and good agreement with expected values is found. The result indicates that the method may be useful for measuring the local magnetic field and in particular, the poloidal component of the field, in a tokamak.