Abstract: A performance monitor system monitors a number of errors occurring in data processed in a communication apparatus having a duplex configuration formed of two systems. The performance monitor system includes a first unit for monitoring a number of errors occurring in data processed in one of the two systems a second unit for monitoring a number of errors occurring in data processed in another one of the two systems and a control unit for controlling operations of the first and second units. Each of the first and second units has a first counter outputting a first error count value and a second counter outputting a second error count value. The control unit controls the first and second counters so that the sum of the first and second error count values obtained in the first unit is equal to sum of those obtained in the second unit.