In underground mines and tunnels accurate and continuous positioning of moving machines and humans has been studied extensively. Geomagnetic field based positioning is an interesting possibility for this measurement need. Using a commercial smart phone and a new calculation service provided by a Finnish company, a feasibility and accuracy assertion tests were done in Finnish underground mine. First time a standard mobile phone, as far as we know, was used in underground tunnel environment to produce continuous position data and an accuracy assessment was performed. An Lshaped tunnel area, two tunnels crossing, was selected for the experiments. A total station was used for reference measurements. Accuracy of measurement in different static and dynamic situations was studied. The found RMSE of 7.32 meters should satisfy many needs. The use of standard smart phone for underground positioning worked fine, but more development work is needed it to be usable in everyday mining situations.