High quality bowel cleaning preparation was most important prerequisites of a accurate colonoscopy, because even a small amount of residual fecal matter can obscure small polyps and increase unnecessary procedure time. Until now, several studies evaluated the impact of bowel preparation on the quality of colonoscopy using comparison of the polyp detection rate in patients with adequate bowel preparation status to that in patients with inadequate bowel preparation status during colonoscopy. However, there was no direct measurement the effect bowel preparation status on the polyp missing rate, the quality of colonoscopy, using tandem colonoscopic evaluation in prospective setting.