A seat main body is mounted on a seat mount base on a vehicle body floor through left and right seat rails. The seat mount base includes left and right side members and a connection member connecting substantially central portions of the side members to each other. The seat rails each include a lower rail and an upper rail. Force sensors are disposed between the side members and the corresponding lower rails. The upper rails are secured to the seat main body. A central region of the connection member, located substantially in a center in a seat width direction, protrudes toward the front compared to joining portions where the connection member is joined to the left and right side members. A space opening toward the rear is formed between the joining portions.
