A vehicle seat structure provided with a seat cushion and a seat back , wherein the seat back includes an upper thoracic support section for supporting at least a portion (Xa) corresponding to a lower part of the upper thoracic spine of a seated occupant (D) having the same size as an AM 50 being a human dummy equal to 50 percentile of an American male adult in size, a lower thoracic support section disposed on a front side with respect to the upper thoracic support section and configured to support a portion (Xb) corresponding to the lower thoracic spine of the seated occupant (D), and a lumbar support section disposed on the front side with respect to the lower thoracic support section and configured to support a portion (Xc) corresponding to the lumbar spine of the seated occupant (D), the lower thoracic support section is configured, when backward inertia force acts on the seated occupant (D), to displace backward greatly, as compared with the upper thoracic support section and the lumbar support section according to the inertia force in such a way that the upper thoracic spine of the seated occupant (D) is inclined forward, and a spring constant (kb) of the lower thoracic support section is lower than a spring constant (ka) of the upper thoracic support section and a spring constant (kc) of the lumbar support section , characterized in that the upper thoracic support section includes, in an inner area thereof, a lower end support section for supporting a seventh thoracic (T7) being a lower end of the upper thoracic spine, and a spring constant (ke) of the lower end support section is lower than a spring constant (ka') of a part of the upper thoracic support section other than the lower end support section . the seat back includes an upper thoracic support section for supporting at least a portion (Xa) corresponding to a lower part of the upper thoracic spine of a seated occupant (D) having the same size as an AM 50 being a human dummy equal to 50 percentile of an American male adult in size, a lower thoracic support section disposed on a front side with respect to the upper thoracic support section and configured to support a portion (Xb) corresponding to the lower thoracic spine of the seated occupant (D), and a lumbar support section disposed on the front side with respect to the lower thoracic support section and configured to support a portion (Xc) corresponding to the lumbar spine of the seated occupant (D), the lower thoracic support section is configured, when backward inertia force acts on the seated occupant (D), to displace backward greatly, as compared with the upper thoracic support section and the lumbar support section according to the inertia force in such a way that the upper thoracic spine of the seated occupant (D) is inclined forward, and a spring constant (kb) of the lower thoracic support section is lower than a spring constant (ka) of the upper thoracic support section and a spring constant (kc) of the lumbar support section , characterized in that the upper thoracic support section includes, in an inner area thereof, a lower end support section for supporting a seventh thoracic (T7) being a lower end of the upper thoracic spine, and a spring constant (ke) of the lower end support section is lower than a spring constant (ka') of a part of the upper thoracic support section other than the lower end support section . <EOS>