In this paper, we analyze empirically software quality changes caused by long-term improvement activities for software review process in a certain company. The objective of our position paper is to present following three points: (1) the review process improvement activity introduced in a company, (2) confirmation that review improvement is fixed in the company (3) evaluation of the effects with respect to the software quality changes caused by the review improvement. Generally, the effectiveness of review process is evaluated from the viewpoint of high productivity of the software process. But truely speaking, customers really concern about the quality of final software product rather than productivity. Thus we analyze the quality of the final product(post-release failure), and show that the effectiveness of review process improvement acticities based on the analysis result.