A floor mat fastening structure, comprising: a seat provided on a vehicle floor in a manner such that said seat can be slid back and forth through seat rails each including an upper rail and a lower rail, a front side of the lower rail of each seat rail being supported by a foot bracket laid on a vehicle floor, by a base end portion of the foot bracket fixed to the lower rail and a front end portion of the foot bracket provided with a fastener fastened on the vehicle floor, a mat main body (3, 3A) laid on the vehicle floor, and two sheet-shaped fastening members integrally including a joining portion and an insertion portion, wherein upper and lower surfaces of the fastening member are flat, the joining portion of each fastening member is coupled and fixed to a lower surface of said mat main body (3, 3A) on the vehicle floor side, with the upper surface of the joining portion substantially being in close contact with the lower surface of said mat main body (3, 3A), the insertion portions of said fastening members are protruding from each of both left and right corner regions of a rear circumferential edge of said mat main body (3, 3A) on said seat side thereof and are C-shaped to form a cutout opening section, each fastening member is fixed to said mat main body (3, 3A) through a rivet member, the rivet member includes a shaft main body and a large-diameter head portion that is provided on an upper portion of the shaft main body and is formed into a shape of a plate, an upper surface of the large-diameter head portion is formed into a curved surface, at least one through hole matched to a through hole of each fastening member is bored in said mat main body (3, 3A), the shaft main body is to be inserted through the through hole and the through hole, and a rivet portion is arranged along a lower surface of each fastening member followed by squashing a lower portion of the shaft main body such that a diameter thereof is formed to expand, wherein the cut-out opening section of the insertion portion has the rear end side of said insertion portion opened, the fastener includes a front end section substantially formed into the shape of a circle when viewed from top and a narrow width section having a width narrower than the front end section and being narrowed from the front end side toward the base end side, that said insertion portion is sandwiched in between the upper surface of the vehicle floor and the bottom of the front end section of said foot bracket, and said fastening member is horizontally pivotally coupled to said mat main body (3, 3A). a seat provided on a vehicle floor in a manner such that said seat can be slid back and forth through seat rails each including an upper rail and a lower rail, a front side of the lower rail of each seat rail being supported by a foot bracket laid on a vehicle floor, by a base end portion of the foot bracket fixed to the lower rail and a front end portion of the foot bracket provided with a fastener fastened on the vehicle floor, a mat main body (3, 3A) laid on the vehicle floor, and two sheet-shaped fastening members integrally including a joining portion and an insertion portion, wherein upper and lower surfaces of the fastening member are flat, the joining portion of each fastening member is coupled and fixed to a lower surface of said mat main body (3, 3A) on the vehicle floor side, with the upper surface of the joining portion substantially being in close contact with the lower surface of said mat main body (3, 3A), the insertion portions of said fastening members are protruding from each of both left and right corner regions of a rear circumferential edge of said mat main body (3, 3A) on said seat side thereof and are C-shaped to form a cutout opening section, each fastening member is fixed to said mat main body (3, 3A) through a rivet member, the rivet member includes a shaft main body and a large-diameter head portion that is provided on an upper portion of the shaft main body and is formed into a shape of a plate, an upper surface of the large-diameter head portion is formed into a curved surface, at least one through hole matched to a through hole of each fastening member is bored in said mat main body (3, 3A), the shaft main body is to be inserted through the through hole and the through hole, and a rivet portion is arranged along a lower surface of each fastening member followed by squashing a lower portion of the shaft main body such that a diameter thereof is formed to expand, wherein the cut-out opening section of the insertion portion has the rear end side of said insertion portion opened, the fastener includes a front end section substantially formed into the shape of a circle when viewed from top and a narrow width section having a width narrower than the front end section and being narrowed from the front end side toward the base end side, that said insertion portion is sandwiched in between the upper surface of the vehicle floor and the bottom of the front end section of said foot bracket, and said fastening member is horizontally pivotally coupled to said mat main body (3, 3A).