To provide a fastening structure of a seat back frame of a vehicle seat capable of efficiently reinforcing the surroundings of fastening bolts in lower parts of both of right and left side frame parts while reducing weight of the seat back frame.

At least both of right and left side frame parts 30 of the seat back frame 21 are formed by a resin material. Lower parts of both the right and left side frame parts 30 are fastened to upper arms 62 of a reclining mechanism 60 with fastening bolts 70 and 71. In parts where the fastening bolts 70 and 71 are inserted in the lower parts of both the right and left side frame parts 30, metal cylindrical members 50 and 51 which include bolt insertion holes and are formed into cylindrical shapes are disposed. The fastening bolts 70 and 71 are inserted into each bolt hole of the upper arms 62 and each of the bolt insertion holes of the metal cylindrical members 50 and 51 and are fastened to fasten the lower parts of both the right and left side frame parts 30 to the upper arms 62.