A detector includes a first flexible sheet, a second flexible sheet placed opposite to the first flexible sheet, and an insulation sheet intervening between them. The first flexible sheet includes a second antenna unit and first contact portions formed on a first opposing surface. The second flexible sheet includes a detection unit and second contact portions formed on a second opposing surface. The first contact portions and the second contact portions become a state of contact by an external force based on the seating of an occupant, cause the second antenna unit to output a transmission signal to the detection unit, and cause the detection unit to output a detection signal to the second antenna unit.