A flange connection structure (100A;200A) including: a first flange (102,104) formed in a pipe (101,103) for flowing a fluid therethrough, and a second flange (102,104) formed in another pipe (101,103) for flowing the fluid therethrough, wherein the flanges (102,104) are fastened by bolts, whereby flange surfaces (102a,104a) of the flanges (102,104) are joined together to couple the pipes (101,103) together, or a first flange formed in a manhole of a tank for storing a fluid and a second flange formed in a cover, wherein the flanges (202,204) are fastened by bolts, whereby flange surfaces (202a,204a) of the flanges (202,204) are joined together to close the manhole with the cover, or a first flange formed in a pipe and a second flange formed in a cover, wherein the flanges (202,204) are fastened by bolts, whereby flange surfaces (202a,204a) of the flanges (202,204) are joined together to close the pipe with the cover, the flange connection structure (100A;200A), comprising: an annular seal groove (105;205) formed in the flange surface (102a,104a;202a,204a) of one of the flanges (102,104;202,204); a pouring passage (106;206) formed in the one of the flanges (102,104;202,204) for bringing the seal groove (105;205) and an outer surface of the one of the flanges (102,104;202,204) into communication with each other; characterized in that a discharge passage (107-1;207-1) is formed in the one of the flanges (102,104;202,204) for bringing the seal groove (105;205) and a space, which is located on an inner peripheral side of the flange and in which the fluid is present, into communication with each other; and sealing liquid pouring means are connected to the pouring passage (106;206) for pouring a sealing liquid into the seal groove (105;205). a first flange (102,104) formed in a pipe (101,103) for flowing a fluid therethrough, and a second flange (102,104) formed in another pipe (101,103) for flowing the fluid therethrough, wherein the flanges (102,104) are fastened by bolts, whereby flange surfaces (102a,104a) of the flanges (102,104) are joined together to couple the pipes (101,103) together, or a first flange formed in a manhole of a tank for storing a fluid and a second flange formed in a cover, wherein the flanges (202,204) are fastened by bolts, whereby flange surfaces (202a,204a) of the flanges (202,204) are joined together to close the manhole with the cover, or a first flange formed in a pipe and a second flange formed in a cover, wherein the flanges (202,204) are fastened by bolts, whereby flange surfaces (202a,204a) of the flanges (202,204) are joined together to close the pipe with the cover, the flange connection structure (100A;200A), comprising: an annular seal groove (105;205) formed in the flange surface (102a,104a;202a,204a) of one of the flanges (102,104;202,204); a pouring passage (106;206) formed in the one of the flanges (102,104;202,204) for bringing the seal groove (105;205) and an outer surface of the one of the flanges (102,104;202,204) into communication with each other; an annular seal groove (105;205) formed in the flange surface (102a,104a;202a,204a) of one of the flanges (102,104;202,204); a pouring passage (106;206) formed in the one of the flanges (102,104;202,204) for bringing the seal groove (105;205) and an outer surface of the one of the flanges (102,104;202,204) into communication with each other; characterized in that a discharge passage (107-1;207-1) is formed in the one of the flanges (102,104;202,204) for bringing the seal groove (105;205) and a space, which is located on an inner peripheral side of the flange and in which the fluid is present, into communication with each other; and sealing liquid pouring means are connected to the pouring passage (106;206) for pouring a sealing liquid into the seal groove (105;205).