Fishing boats have several restrictions on their machinery arrangements and space utility on deck. For the improvement of the stability and the deck utility, the fitting of a bulge on sidewall of the vessel is one of the solutions. However, the bulge generally makes a significant increase of hull resistance and loss of speed. At the principal design stage of the elongation of the fisheries research ship “Ushio-maru”, the University of Hokkaido, the authors examined the suitable form of the bulge to improve the stability without increase of the hull resistance. For this purpose, systematic calculations of wave making resistance by Rankine’s source method and model experiments using a circulating water channel have been conducted. In this process, it has been found that the optimized bulge form can decrease the hull resistance. In this paper, the authors discuss about the mechanism of the hull resistance decrease by a bulge based on the view point of flow field around hull and propose the suitable method to design the bulge form. This technique has been applied to the actual elongation design of “Ushio-maru”, and then it has been confirmed by the experimental model tests as well as the full scale trials.