A plug structure for sealing an opening end portion of a circular fluid passage formed in a housing comprises a plug being equal in resistance against deformation to, or smaller than, the housing and having an annular flange portion formed on an end portion of a circular column portion; a flange pressure-contacting portion of the opening end portion deformed to be pressure-contacted with a lower surface of the annular flange portion by being caulked with a caulking tool; a contracted diameter portion of a shape gradually narrow in the middle to which the circular column portion is deformed as a result that the fluid passage is deformed by the caulking to be contracted in diameter; and a deformed circumferential portion to which the circumferential edge of the annular flange portion of the plug is deformed by the caulking to be pressure-contacted with the flange pressure-contacting portion of the opening end portion.