A seat back board includes: a flexible plate member which is made of a resin material and has the Shore hardness higher than 50, the plate member including: a back-surface covering part which covers a back surface of a seat back; and a bottom-surface covering part which covers a bottom surface of a seat cushion; a sewing part, which is sewn with a cover member covering a front surface and side surfaces of the seat back or is sewn with a coupling member to be coupled with the cover member in a detachable manner, being provided in an edge portion of the back-surface covering part; and a locking part, to be locked in the seat cushion, being provided in the bottom-surface covering part.