This paper discusses the effects of the estimation accuracy for software development cost on both the quality of the delivered codes and the productivity of the development team. The estimation accuracy is measured by metric RE(called relative error). Similarly, the quality and productivity are measured by metrics FQ(field quality) and TP(team productivity). Using actual project data on thirty-one projects at a certain company, the followings are verified by correlation analysis and test of statistical hypotheses: (1) There is a high correlation between the faithfulness of development plan to standards and the value of RE(A coefficient of correlation between them is -0.60). (2) Both FQ and TP are significantly different between projects with -10%