Employment Social Security Agency is one of the public legal entities in Indonesia. This entity has to protect all workers through employment social insurance programs. To keep its effectiveness of services for the public, Employment Social Security Agency uses IT outsourcing in managing the IT services. When Employment Social Security Agency acquires IT services externally, it will engage in relationships with a service provider. As an IT service manager, the IT division has to be able to ensure the successful implementation of IT outsourcing and support business processes. Thus, the aim of this research is to identify critical success factors within the public sector's IT outsourcing in Indonesia. The Analytical Hierarchy Process (AHP) method was used to analyze and rank the success factor. There are 14 factors evaluated, which categorized as Organizational Environment, Contract/Project Characteristics, Project Management, Partnership Management category. Based on the highest rank in each category, showed that the most important factor is Top Management Support, Project Management Skill, Knowledge Transfer and Project Budget and Size.