In recent years, the architectural design process has been becoming more and more complicated due to increase in factors to consider. Therefore, facilitation in the design process is becoming more important to reduce the task and needlessness work. As a tool to facilitate design process, BIM (Building Information Modeling) is the focus of attention recently. But, in spite of the large expectations for BIM's influence of facilitating design work, actual introduction of BIM into the building industry is very few, because of the lack of quantitative evaluation of the effectiveness of BIM. Therefore, in this research, we evaluated the effectiveness of BIM utilization on reducing reworks and improving the design flow by comparing the 7 buildings which designed with BIM and 3 buildings which designed without BIM.<br> As preparation, we organized meeting date and contents in chronological order from minutes information of 10 buildings. And we classified the contents into 113 design work items.<br> The first analysis is the evaluation using a progress chart. It is the graph showing the progress of the design progress degree. The average of the design progress degree in BIM used cases was higher at the early stage of design process than that of the BIM unused cases. So that, it made clear that many design work items are considered from the early stage of the design process in BIM used cases.<br> The second analysis is the evaluation of the study timing of design work items. It is the percentage value expressed as the average timing of the meeting. The study timing in BIM used cases average is 28%, and that of study timing in BIM unused cases average is 39%. which shows that utilization of BIM leads forward the meeting timing. Next, analysis of the study timing of 19 design work group shows that “16. Structural performance” and “19. Estimate and make the book of confirmation request” were studied early stage of design process. And, analysis of the study timing of 113 design work items shows that “46. Decide the beam size”, “78. Study the fire protection zone”, “79. Design the corridors”, “94. Decide the floor vibration performance”, “103. Design the window (form/kind)” and “113. Make the book of construction order” were studied early stage of design process.<br> The third analysis is the evaluation of rework. We clarify the type of rework occurred at each meeting. The type is classified as “1. Rework by owner's change request”, “2. Returning by designer's change request”, “3. Rework by the change of other design work item” and “4. The other”. In this research, the rework frequency of BIM used cases is 28% in average, and that of BIM unused cases is 19% in average. It shows that reworks are increased 9% by BIM using. However, in the interview survey, designer said “Utilizing BIM, the cases that have been overlooked in the design stage and become troubles at the construction stage have been discovered and improved at the design stage.”.<br> In conclusion, using BIM in the architectural design process is effective in front-loading of design work, but it becomes a factor to increase the rework of design work. However, the number of building in this research is 10, making it difficult to say statistical analysis results. Also, if the increase in the rework at the basic design stage is one that foresaw the trouble at the construction stage and avoided it, verification including implementation design is required.