A frame transmitting unit divides a payload area of an upper-layer frame into a plurality of slots, maps a lower-layer frame to one or more of the slots, and transmits the upper-layer frame including connection information that indicates which one or more of the slots the lower-layer frame of a same type is mapped to. The frame synchronization detection unit identifies, as one or more allocated slots, the one or more slots to which the lower-layer frame of the same type is mapped, on the basis of the connection information, and detects the frame synchronization by identifying a frame head signal of the lower-layer frame of the same type that is mapped to the one or more allocated slots.