A drain plug structure for an oil pan, which is a detachable drain plug structure for closing off a drain hole formed in a bottom portion of the oil pan, said drain plug structure being characterized in that it comprises: a columnar stopper portion which is inserted into the drain hole and rotated in order to prevent withdrawal; a sealing member fitted to an outer periphery of the columnar stopper portion; an operating portion for the abovementioned rotation operation, which is formed on an outside exposed end portion of the columnar stopper portion; a plurality of cantilevered arcuate arm portions which are formed projecting radially outwards from the outside exposed end portion of the columnar stopper portion and are also arranged in series along a circumferential direction; and a loosening-prevention meshing portion (16,17,18,19) which is formed on both a tip end portion of each arcuate arm portion and an opening edge portion of the drain hole, and demonstrates a loosening-prevention function by fitting together in a recess/projection engagement; characterized by, a seating flange portion which is formed as a single piece with one end portion of the columnar stopper portion in such a way as to project radially therefrom, and is larger in diameter than said one end portion and has a top portion of predetermined thickness; wherein the seating flange portion has a shape which is equally divided by means of four slot grooves having a roughly deformed "<" shape such as to leave bridge portions at four locations in the circumferential direction, and said seating flange portion is slotted in a radial direction and the circumferential direction as to form four arcuate arm portions which are cantilevered and have a circular arc shape, arranged in series along the circumferential direction, while the four bridge portions remain as a root portion; wherein the circumferential length of the arcuate arm portions is sufficiently large to provide adequate flexibility in a thickness direction with the bridge portions as a support point, and the structure enables elastic deformation based on the elastic strength thereof in the thickness direction; wherein the peripheral length and wall thickness of the arcuate arm portions are set in such a way as to take account of the torque required when the drain plug is removed (loosened) and the amount of flexing and deformation of the arcuate arm portions required to engage/disengage the engaging projections (16,19) and the engaging recesses (17,18), so the operations do not lead to damage and repeated usage is possible. a columnar stopper portion which is inserted into the drain hole and rotated in order to prevent withdrawal; a sealing member fitted to an outer periphery of the columnar stopper portion; an operating portion for the abovementioned rotation operation, which is formed on an outside exposed end portion of the columnar stopper portion; a plurality of cantilevered arcuate arm portions which are formed projecting radially outwards from the outside exposed end portion of the columnar stopper portion and are also arranged in series along a circumferential direction; and a loosening-prevention meshing portion (16,17,18,19) which is formed on both a tip end portion of each arcuate arm portion and an opening edge portion of the drain hole, and demonstrates a loosening-prevention function by fitting together in a recess/projection engagement; characterized by, a seating flange portion which is formed as a single piece with one end portion of the columnar stopper portion in such a way as to project radially therefrom, and is larger in diameter than said one end portion and has a top portion of predetermined thickness; wherein the seating flange portion has a shape which is equally divided by means of four slot grooves having a roughly deformed "<" shape such as to leave bridge portions at four locations in the circumferential direction, and said seating flange portion is slotted in a radial direction and the circumferential direction as to form four arcuate arm portions which are cantilevered and have a circular arc shape, arranged in series along the circumferential direction, while the four bridge portions remain as a root portion; wherein the circumferential length of the arcuate arm portions is sufficiently large to provide adequate flexibility in a thickness direction with the bridge portions as a support point, and the structure enables elastic deformation based on the elastic strength thereof in the thickness direction; wherein the peripheral length and wall thickness of the arcuate arm portions are set in such a way as to take account of the torque required when the drain plug is removed (loosened) and the amount of flexing and deformation of the arcuate arm portions required to engage/disengage the engaging projections (16,19) and the engaging recesses (17,18), so the operations do not lead to damage and repeated usage is possible.