There was elaborated a method for calculating magnetic fields of the Solar System planets. It is based on the quantum theory of electroconductivity of metals and semiconductors. The latter helps to calculate thermoelectrical processes, always taking place in the bowels of “hot” planets. Main elements of those processes are planetary temperature gradients, thermo electromotive force and radially directed thermoelectrical currents, which are associated with Seebeck effect. Thermo electromotive force causes directional movement of planetary thermoelectrical currents both in metal cores and other conductive shells of planets. Those currents are big and they generate magnetic fields of proportional intensity. The capacities of the calculation method were tested while finding the reason why the Jupiter magnetic field is such complicated. As a result it was specified that the source of the main magnetic field of a planet is its metal core and the source of an additional magnetic field is the layer of liquid metal hydrogen. There was also found the third source of a local magnetic field of low intensity along the circular zone of the equatorial region. The conclusion that the Jupiter’s main magnetic field has a polarity opposite to the Earth’s one.