A fastening portion structure of a vehicle, comprising: a collar (30, 52, 68, 70) including a cylindrical shape, the collar (30, 52, 68, 70) including an end surface on one side in an axial direction of the collar (30, 52, 68, 70) and the other end in the axial direction of the collar (30, 52, 68, 70), the end surface on one side in an axial direction of the collar (30, 52, 68, 70) being arranged farther toward a vehicle upper side than the other end in the axial direction of the collar (30, 52, 68, 70); a first plate portion (20, 58), the end surface on the one side in the axial direction of the collar (30, 52, 68, 70) abutting against the first plate portion (20, 58), the first plate portion (20, 58) including a first hole being formed in a position overlapping with a through-portion of the collar (30, 52, 68, 70) through the first plate portion (20, 58); a second plate portion being placed on an opposite side of the first plate portion (20, 58) from a side with the collar (30, 52, 68, 70), the second plate portion including a second hole being formed in a position overlapping with the through-portion of the collar (30, 52, 68, 70) and the first hole of the first plate portion (20, 58) through the second plate portion; a bolt being inserted through the through-portion of the collar (30, 52, 68, 70), the first hole of the first plate portion (20, 58), and the second hole of the second plate portion; a nut (38, 62) applying a fastening load to the collar (30, 52, 68, 70), the first plate portion (20, 58), and the second plate portion, together with the bolt, by the bolt being screwed to the nut (38, 62); and characterized by a protruding portion (42, 60) being formed on at least one of an end portion on the one side in the axial direction of the collar (30, 52, 68, 70) and an end portion on the side of the first plate portion (20, 58) with the first hole, the protruding portion (42, 60) protruding toward the second hole of the second plate portion. a collar (30, 52, 68, 70) including a cylindrical shape, the collar (30, 52, 68, 70) including an end surface on one side in an axial direction of the collar (30, 52, 68, 70) and the other end in the axial direction of the collar (30, 52, 68, 70), the end surface on one side in an axial direction of the collar (30, 52, 68, 70) being arranged farther toward a vehicle upper side than the other end in the axial direction of the collar (30, 52, 68, 70); a first plate portion (20, 58), the end surface on the one side in the axial direction of the collar (30, 52, 68, 70) abutting against the first plate portion (20, 58), the first plate portion (20, 58) including a first hole being formed in a position overlapping with a through-portion of the collar (30, 52, 68, 70) through the first plate portion (20, 58); a second plate portion being placed on an opposite side of the first plate portion (20, 58) from a side with the collar (30, 52, 68, 70), the second plate portion including a second hole being formed in a position overlapping with the through-portion of the collar (30, 52, 68, 70) and the first hole of the first plate portion (20, 58) through the second plate portion; a bolt being inserted through the through-portion of the collar (30, 52, 68, 70), the first hole of the first plate portion (20, 58), and the second hole of the second plate portion; a nut (38, 62) applying a fastening load to the collar (30, 52, 68, 70), the first plate portion (20, 58), and the second plate portion, together with the bolt, by the bolt being screwed to the nut (38, 62); and characterized by a protruding portion (42, 60) being formed on at least one of an end portion on the one side in the axial direction of the collar (30, 52, 68, 70) and an end portion on the side of the first plate portion (20, 58) with the first hole, the protruding portion (42, 60) protruding toward the second hole of the second plate portion.