The traditional reliability design methods are imperfect, because the designed systems aim at less faults but once a fault happens they may hard- fail. To solve this problem, we propose the con- cepts of self-maintenance machine (SMM) that can maintain its functions flexibly even though faults happen. In order to achieve capabilities of diagnosing and repair planning, a model based approach that employs qualitative physics is proposed. Regarding repair executing capability, control type repair strategy is proposed. A prototype of the self-maintenance machine is devel- oped and it succeeded to maintain its functions as far as its structure does not change. This prototype, however, revealed the following problems that may arise when applied to a commercial product as embedded software. The problems include; (1)performance of the reasoning system, (2)the system size, (3)effects of environmental changes, and (4) roughness of qualitative repair operations. In order to solve these problems, we propose a new reasoning method based on vir- tuul cases and fuzzy qualitative values. This is a methodology of knowledge compilation that gives better reasoning performance and can deal with real world applications like the self-maintenance machine. By using this method, we finally developed a commercial photocopier that has self-maintenability and is more robust against faults. The commercial version has been supplied world-wide as a product of Mita Industrial Co., Ltd., since April, 1994.