While a detailed cost estimation utilizes the comprehensive quantity information obtained from a complete drawing and specification, an approximate cost estimation heavily relies on the subjective decisions. This phenomenon is because the approximate estimation is usually conducted in early stage of the construction phase where the drawing and specification are not available. This study developed a roadway construction cost estimation system which enables users to estimate the approximated cost of the project in early stage. The system identifies the variation of the construction costs due to the road route change using a cased based reasoning(CBR) method. Overall the system is expected to improves the accuracy and effectiveness of the estimation process, because it would provide users with increased accessability and consistent results in early stage of the project which leaded to the increased work transparency.