A frame generating apparatus that maps a client signal into an optical data transfer unit frame with a higher bit rate than the client signal, the frame generating apparatus comprising: a deserializer configured to deserialize the client signal into parallel signals, the number of parallel signals corresponding to the number of tributary slots of the optical data transfer unit frame; a plurality of generic mapping procedure circuits configured to insert data of the client signal and stuff into a payload area of the optical data transfer unit frame based on a difference between a bit rate of the client signal and a bit rate of the optical data transfer unit frame; and a serializer configured to serialize the parallel signals output from the plurality of generic mapping procedure circuits, wherein the frame generating apparatus is configured to map a group of M successive bytes of the client signal into a group of M successive bytes of the optical data transfer unit frame, where M is the number of the tributary slots; and wherein the frame generating apparatus inserts the data of the client signal into the payload area of the optical data transfer unit frame when a first expression (N×Cn) mod Tb<Cn is satisfied, and inserts the stuff into the payload area of the optical data transfer unit frame when a second expression (N×Cn) mod Tb ≥ Cn is satisfied, "N" representing address assigned to the payload area of the optical data transfer unit frame, "Cn" representing (the bit rate of the client signal)÷(the bit rate of the optical data transfer unit frame)×(total number of bytes of the payload area of the optical data transfer unit frame), "Tb" representing the total number of bytes of the payload area of the optical data transfer unit frame, and "mod" representing a remainder operator. a deserializer configured to deserialize the client signal into parallel signals, the number of parallel signals corresponding to the number of tributary slots of the optical data transfer unit frame; a plurality of generic mapping procedure circuits configured to insert data of the client signal and stuff into a payload area of the optical data transfer unit frame based on a difference between a bit rate of the client signal and a bit rate of the optical data transfer unit frame; and a serializer configured to serialize the parallel signals output from the plurality of generic mapping procedure circuits, wherein the frame generating apparatus is configured to map a group of M successive bytes of the client signal into a group of M successive bytes of the optical data transfer unit frame, where M is the number of the tributary slots; and wherein the frame generating apparatus inserts the data of the client signal into the payload area of the optical data transfer unit frame when a first expression (N×Cn) mod Tb<Cn is satisfied, and inserts the stuff into the payload area of the optical data transfer unit frame when a second expression (N×Cn) mod Tb ≥ Cn is satisfied, "N" representing address assigned to the payload area of the optical data transfer unit frame, "Cn" representing (the bit rate of the client signal)÷(the bit rate of the optical data transfer unit frame)×(total number of bytes of the payload area of the optical data transfer unit frame), "Tb" representing the total number of bytes of the payload area of the optical data transfer unit frame, and "mod" representing a remainder operator.