The purpose of this study was to suggest a guideline for systematic and practical critical success factors of the External IT audit. For this purpose, an integrated conceptual model is developed considers that the service quality, and the IS(Information Systems) success with a contingency viewpoint. The model is tested using 254 data of IS projects which procured the external IT auditing service in public sector of Korea. The results of this study are as follows. First, service quality attributes of responsiveness, reliability, and tangibility have positive influence on IS implementation process quality. Second, the external IT audit quality obviously contributes to IS implementation success considering both IS implementation process quality and IS system quality. However it has positive influence on the process quality directly and the system quality indirectly.