1. A vehicular electronic control apparatus comprising: a mother unit including a microcomputer; a plurality of ECU modules mounted to the mother unit detachably for performing different controls for a vehicle, respectively; an operation data communication section for communicating operation data, which the plurality of ECU modules uses in performing operation processing for the respective controls, with the microcomputer; an alternative operation processing section for causing the communication section to transmit the operation data to at least one of the microcomputer and one ECU module of the plurality of ECU modules through the microcomputer, and for causing the microcomputer or the one ECU module, to which the operation data is transmitted, to perform alternative operation processing corresponding to at least a part of operation processing, which is to be executed by another ECU module of the plurality of ECU modules; and an ECU load check section configured to check whether each of the plurality of ECU modules is in a high processing load state, wherein the alternative operation processing section is configured to determine the another ECU module to be an ECU module, which is determined to be in the high processing load state, so that the alternative operation processing is performed by the at least one of the microcomputer and one ECU module in place of the ECU module determined to be in the high processing load state.