Obesity has been recognized as important risk factors for colorectal cancer. However, limited evidence is available on colorectal cancer and body mass index (BMI) in Asian population. We conducted a pooled analysis of eight population-based prospective cohorts studies in Japan with more than 300000 subjects to evaluate an impact of obesity in terms of BMI on colorectal cancer risk with unified categories. We estimated summary hazard ratio (HR) by pooling of study-specific HR for BMI categories with random effect model. We found a significant positive association between BMI and colorectal cancer risk in male and female. Adjusted HRs for 1 kg/m2 increase were 1.03 95% confidence interval (CI) 1.02–1.04 for males and 1.02 (95% CI 1.00–1.03) for females. The association was stronger in colon, especially in proximal colon, relative to rectum. Males showed a stronger association than females. Population attributable fraction for colorectal cancer by BMI ≥25 kg/m2 was 3.62% (95% CI 1.91–5.30) for males and 2.62% (95% CI 0.74–4.47) for females. We found significant association between BMI and colorectal cancer risk by pooling of data from cohort studies with considerable number of subjects among Japanese population. This information is important in cancer control planning, especially in Asian population.