Cost budgeting is a fundamental component of cost management, establishing a baseline for building construction projects. However, previous studies suggest that the cost budgeting at most construction companies are not systematically conducted. The purpose of the present study is to reveal the key factors which influence cost budgeting and to weight these key factors related to building construction projects. A questionnaire survey for identifying the factors which influence cost budgeting was conducted with cost budgeting experts. The analytic hierarchy process was employed to weight the relative importance of the factors identified. The results showed that data is most important category and level of design documents is most important factor. However, gaps of importance between factors are small, so it is revealed that all of 35 factors are influencing the cost budgeting. Research for using these weights of key factors influencing the cost budgeting is needed to quantify the level of cost risk during cost budgeting process, which enables cost planners and management to use improving the reliability of cost budgeting for building construction projects.