Flower arrangement is enjoyed all over the world as a way of gardening and in decoration of rooms. However, it is difficult to make arrangements as beautiful as possible for beginners. Some rules about layout and color combination of flower arrangement are required in order for beginners to beautifully arrange the flowers. Moreover, the process is sometimes un-reversible; that is, if a user cuts a stem too short, it may irrevocably spoil the arrangement. To solve these problems, we propose a computer-supported flower arrangement simulator, "CADo", which helps beginners enjoy flower arrangements using familiar materials. The name CADo is a combination of Kado (the Japanese art of flower arrangement) and CAD (computer aided design). CADo helps users with arrangement suggestions based on traditional layout and color combination of flower arrangement, and instructs users on how to cut flowers and to arrange them step by step. First, we performed a feasibility study to implement the proper rules of flower arrangement to our system, after which the system was built and developed. Finally, we verified the effectiveness of our system through evaluative experiment.