Objective : Risk factors for SSI after open heart surgeries were evaluated in relation to a bundle of SSI preventive measures. Methods : Research design is a retrospective cohort study. The study population was 1,579 patients who had received open heart surgeries at Kobe City Medical Center General Hospital from January 2008 to December 2010 (Period I : when standard infection prevention measures were implemented) and from January 2014 to December 2016 (Period II : after a relocation of the hospital to a new campus and enhanced infection prevention measures were implemented). Factors associated with SSI were determined using univariate modelling analysis followed by multi-variate logistic regression analysis. The Center for Disease Control and Prevention definition of SSI was used for case determination. Results : Overall SSI incidence was 4.5%. SSI incidence decreased significantly from 6.6% in Period I to 2.9% in Period II (p＜0.001). Significant improvement in adherence to the recommended preventive measures was observed in Period II in selection of appropriate antibiotics, discontinuation of prophylactic antibiotics within 72 h after surgery and glucose control on post-operative Day 1 and 2 (p＜0.001). A univariate analysis showed statistical significance in surgical procedure, surgical period, surgical duration, post-operative day 2 morning glucose level, administration of prophylactic antibiotics within 1 h before incision, 100% compliance with the Bundle. Complex surgery (odds ratio 2.5 ; 95%CI 1.3～4.8) were identified as a risk factor by multiple logistic regression. Surgical period (Period II, odds ratio 0.41 ; 95%CI 0.28～30.71) and administration of prophylactic antibiotics within 1 h before incision (odds ratio 0.57 ; 95%CI 0.33～0.97) reduced SSI risks. Conclusion : The study demonstrated administration of prophylactic antibiotics within 1 h before incision was particularly important for SSI prevention. Higher compliance with SSI bundle and a special attention to patients receiving complex surgery were also warranted.