A seat structure. A reinforcing structure in a substantially rectangular frame shape is set in an area where cushion-part side frames and back-part side frames are coupled to each other via reclining mechanism parts, from vicinities of rear portions of the cushion-part side frames up to vicinities of lower portions of the back-part side frames. The reinforcing structure increases unity of a cushion frame and a back frame, and when a load is applied to the back frame due to an impact at the time of a rear-end collision or the like, the load acts as a force deforming mainly the cushion frame via the reinforcing structure, and when the body of a seated person thereafter moves rearward, the load acts as a force deforming mainly the back frame, making it possible to use not only strength of the back frame but also strength of the cushion frame.