An electrolytic corrosion prevention structure of a flange connection section, capable of suppressing the occurrence of electrolytic corrosion without increasing the number of airtight/watertight positions and capable of being easily processed. The electrolytic corrosion prevention structure has pipe members (21, 22) having flanges (21a, 22a) at ends of the pipe members and also having a groove surrounding ends of pipe sections, the groove being present between the flanges (21a, 22a) when the flanges (21a, 22a) are abutted on each other, an annular seal member (23) placed at a position closer to the radially outer ends of the flanges than the position where the flanges are joined to each other, and an interposition member (24) placed on the radially outer side of the seal member (23). The electric potential difference occurring when the material of either of the pipe members (21, 22) and the material of the interposition member (24) are made to contact with each other is smaller than the electric potential difference occurring when the materials of the pipe members (21, 22) are made to contact with each other.