Software is used everywhere in society, and is assuming responsibility for the systems that support the foundation of society. Nevertheless, software vendors are struggling to guarantee quality. To solve the problem, NEC Corporation devised a quality management method called "Software Quality Accounting", which has been built up and operated for over 30 years. NEC has been able to reduce and maintain the number of post-release defects to less than 5%. Quality accounting is based on the idea that defects should be seen as "debts". It is more economical to pay off debts early, before they grow due to accrued interest. Software defects are similar in the sense. This book introduces the methodology of "Software Quality Accounting" and how to apply it. And three actual examples of quality improvements made through the application of software quality accounting are introduced. Also, a supporting framework, developed to help make efficient use of Software Quality Accounting, has been contributing to increases in quality as well. Based on the results, this book identifies the primary factors of success with high quality software development and an overall perspective.