A signal processing method for an ATM switching network formed by connecting a plurality of ATM switching networks in which header conversion tables of line interfaces can be rewritten by control cells, includes in response to occurrence of an abnormality in a call control processor of a ATM switching system A, informing an ATM switching system B of the occurrence of the abnormality, transferring call control information in the ATM switching system A to the ATM switching system B, rewriting header conversion tables included in a plurality of line interfaces of the ATM switching system A by using control cells generated by the ATM switching system A and thereby transferring signal channel cells arriving at the ATM switching system A after occurrence of the abnormality to the ATM switching system B, and rewriting header conversion tables included in a plurality of line interfaces of the ATM switching system A by using control cells generated by the ATM switching system B and thereby making a call control processor of the ATM switching system B effect route control of information channel cells within the ATM switching system A.