An approximate method of computing disturbances of the terrestrial magnetic field on the sea bottom due to a wooden vessel, who is made of so-called non-magnetic materials for the most part and is dotted with magnetic materials on board, is presented theoretically and confirmed with the measured results of an actual vessel. Results of numerical calculations are shown as examples, which have been carried out in the preliminary design of a wooden mine-sweeper, not only on her magnetic field but also on the effectiveness of several kinds of degaussing coils. Electro-magnetic field induced by an electric circuit on board, for instance, the magnetic sweeping cable, is also investigated together with the practicable method of reducing its effects.