Abstract: A path monitoring bit extraction device in a transmission communication system for dividing a information transmission frame into a plurality of blocks and assigning time slots in the blocks to data of a plurality of paths respectively including a path monitoring bit for transmission in a time division multiplexing manner. The path monitoring bit extraction device comprises a readable and writable memory and detection means for detecting multiplexing positions of the path monitoring bits and block start position in the information transmission frame. Write enabling means enables writing in the memory in response to detection timing of the multiplexing position of the path monitoring bit by the detection means.