Abstract: An exchange has units traversed by cells, such as a line interface unit, a cell multiplexer/demultiplexer unit and a cell switch unit, as well as a processor. Each cell traversing unit detects and counts passing cell and cell error on a per-connection basis, adds the counts, along with unit identification data, onto an intra-office cell and then transmits the cell. One of the cell traversing units serving as an end unit accumulates, for each cell traversing unit and on a per-connection basis, the passing cell counts and the error cell counts sent from the other cell traversing units. The processor detects a fault in a cell traversing unit upon acquiring the passing cell count and cell error count, on a per-connection basis, of each cell traversing unit, the passing cell count and cell error count having been accumulated in the cell traversing unit serving as the end unit.