To provide a bolt holding structure for a flange saving labor of fixing a flange and a bolt by welding or the like, and at the same time capable of facilitating insertion of the bolt into a hole part of the counterpart flange by providing the bolt with a freedom of movement to some extent.

A bolt inserting hole part 21 is formed on the flange 20a, and the bolt 23 with a polygonal, for example hexagonal bolt head 22 is inserted in the bolt inserting hole part 21. The bolt head 22 is fit in a planar part 25 to which the flange 20a of a purifier 24 is fixed, and a hexagonal groove part 27 in which parallel parts 26 and 26 mutually facing opposite directions of the bolt head 22 is locked is formed on the planar part 25. In this state, a flange 20b fixed to one end of a pipe 29 is fit on the bolt 23, and the purifier muffler 24 and the pipe 29 can be joined by tightening a nut 31. When the nut 31 is tightened, in the bolt 23, the parallel parts 26 of the bolt head 22 are locked in the groove part 27 of the purifier 24 and rotation of the bolt head 22 is blocked.