An insert nut insertion structure, comprising: an insert nut including a bolt fastening portion having threads on an inner circumference into which a bolt is screwed; and a flange portion provided on an outer circumference of the bolt fastening portion and extended in a circumferential direction of the bolt fastening portion; and a nut fixing groove into which the insert nut is inserted in a first direction perpendicular to a direction in which the bolt is screwed, wherein the nut fixing groove includes: a flange receiving portion having an internal space conforming to a shape of the flange portion; and a pair of fingers (22, 23) provided on the flange portion positioned in the flange receiving portion and formed in a cantilever shape that surrounds, in part, a periphery of the bolt fastening portion, wherein the flange receiving portion includes: a base plate on which the flange portion is seated; two side plates (21b, 21c) which are vertically extended from the base plate and contact two sides of the flange portion; and cover plates (21d, 21e) which are horizontally extended from the two side plates (21b, 21c), contact an upper surface of the flange portion and are spaced a predetermined distance apart from the pair of fingers (22, 23), characterized in that the flange portion includes a lower flange portion and an upper flange portion formed in double layer along a lengthwise direction of the bolt fastening portion, wherein the upper flange portion is positioned in a space between the pair of fingers (22, 23) and the cover plates (21d, 21e), and wherein the lower flange portion is positioned in a space between the cover plates (21d, 21e) and the base plate. an insert nut including a bolt fastening portion having threads on an inner circumference into which a bolt is screwed; and a flange portion provided on an outer circumference of the bolt fastening portion and extended in a circumferential direction of the bolt fastening portion; and a nut fixing groove into which the insert nut is inserted in a first direction perpendicular to a direction in which the bolt is screwed, wherein the nut fixing groove includes: a flange receiving portion having an internal space conforming to a shape of the flange portion; and a pair of fingers (22, 23) provided on the flange portion positioned in the flange receiving portion and formed in a cantilever shape that surrounds, in part, a periphery of the bolt fastening portion, wherein the flange receiving portion includes: a base plate on which the flange portion is seated; two side plates (21b, 21c) which are vertically extended from the base plate and contact two sides of the flange portion; and cover plates (21d, 21e) which are horizontally extended from the two side plates (21b, 21c), contact an upper surface of the flange portion and are spaced a predetermined distance apart from the pair of fingers (22, 23), characterized in that the flange portion includes a lower flange portion and an upper flange portion formed in double layer along a lengthwise direction of the bolt fastening portion, wherein the upper flange portion is positioned in a space between the pair of fingers (22, 23) and the cover plates (21d, 21e), and wherein the lower flange portion is positioned in a space between the cover plates (21d, 21e) and the base plate. a flange receiving portion having an internal space conforming to a shape of the flange portion; and a pair of fingers (22, 23) provided on the flange portion positioned in the flange receiving portion and formed in a cantilever shape that surrounds, in part, a periphery of the bolt fastening portion, wherein the flange receiving portion includes: a base plate on which the flange portion is seated; two side plates (21b, 21c) which are vertically extended from the base plate and contact two sides of the flange portion; and cover plates (21d, 21e) which are horizontally extended from the two side plates (21b, 21c), contact an upper surface of the flange portion and are spaced a predetermined distance apart from the pair of fingers (22, 23), characterized in that the flange portion includes a lower flange portion and an upper flange portion formed in double layer along a lengthwise direction of the bolt fastening portion, wherein the upper flange portion is positioned in a space between the pair of fingers (22, 23) and the cover plates (21d, 21e), and wherein the lower flange portion is positioned in a space between the cover plates (21d, 21e) and the base plate. a base plate on which the flange portion is seated; two side plates (21b, 21c) which are vertically extended from the base plate and contact two sides of the flange portion; and cover plates (21d, 21e) which are horizontally extended from the two side plates (21b, 21c), contact an upper surface of the flange portion and are spaced a predetermined distance apart from the pair of fingers (22, 23), characterized in that the flange portion includes a lower flange portion and an upper flange portion formed in double layer along a lengthwise direction of the bolt fastening portion, wherein the upper flange portion is positioned in a space between the pair of fingers (22, 23) and the cover plates (21d, 21e), and wherein the lower flange portion is positioned in a space between the cover plates (21d, 21e) and the base plate.