To prevent stress concentration on an L-shaped bending part by forming a burring part on a cushion bracket, boring a through-hole in an opposed cushion frame, and performing calking processing in a state of fitting the through-hole of the cushion frame to the burring part.

A cushion bracket 11 has an installing plate part, and a cushion frame 17 of a seat cushion is fixed to this installing plate part. A projecting part 17a is formed on the outer peripheral edge side of the cushion frame 17, and an L-shaped bending part 17b is formed on the inner peripheral edge side. A burring part 31 is formed in the installing plate part of the cushion bracket 11, and a through-hole 33 is bored in a horizontal part 17c of the cushion frame 17 opposed to this burring part 31. Calking processing is performed in a state of fitting the through-hole 33 to the horizontal part 17c of the cushion frame 17 in the burring part 31 to thereby prevent the occurrence of stress concentration on the L-shaped bending part 17b to prevent the occurrence of a crack.