In an address decision system in an ATM exchange, a table memory stores data showing relationships between VPI/VCI values and addresses. A latch circuit latches a VPI/VCI value contained in a cell transferred via a cell highway. A comparator circuit compares the VPI/VCI values stored in the table memory with the VPI/VCI value latched by the latch and generates a comparator output signal showing, in a normal operation, one of the addresses at which the VPI/VCI value from the latch circuit coincides with one of the VPI/VCI values in the table memory. An address decision unit encodes the comparator output signal and generates an encoded signal based on the comparator output signal. A decoder unit decodes the encoded signal and generates a decoded signal. A check unit receives the comparator output signal and the encoded signal and generates an error signal when the comparator output signal and the encoded signal do not match each other.