Background: In nonshockable cardiac arrest (CA) patients, the 2020 cardiopulmonary resuscitation (CPR) guidelines have stressed that high-quality CPR improves survival from CA, as with the 2015 CPR guidelines. However, it is unknown whether the 2015 guidelines contributed to the favorable neurological outcomes in adult CA patients. The present study aimed to clarify the effects of the 2015 guidelines on adult CA patients using the data of the All-Japan Utstein Registry, a prospective, nationwide, population-based registry of out-of-hospital CA (OHCA). Methods: From the data of this registry, between 2011 and 2020, we included adult witnessed OHCA patients due to cardiac etiology who had non-shockable rhythm as an initial rhythm. We excluded patients who received prehospital care in 2011, 2015, 2016, and 2020 because it was difficult to distinguish prehospital care based on either 2010 CPR guidelines, 2015, or 2020. We also excluded patients who received bystander CPR by citizens because we cannot assess the quality of bystander CPR. Study patients were divided into two groups based on the different CPR guidelines; the era of the 2010 guidelines (2010G) and the era of the 2015 guidelines (2015G). The endpoint was the favorable neurological outcome at 30 days after OHCA. Potential confounding factors based on biological plausibility and previous studies were included in the multivariable logistic regression analysis. These variables included the age, sex (male, female), advanced airway or not, the administration of adrenaline or not, the administration of saline or not, and time interval from call EMS to the scene. Results: Of the 1,259,960 patients registered in the All-Japan Utstein Registry, the data of 54,219 patients were included in this analysis. The 2015G was significantly higher in the 30-day favorable neurological outcome than the 2010G (2010G vs. 2015G = 1.5% vs. 1.8%: p=0.008). In the multivariate analysis, the adjusted odds ratio for 30-day favorable neurological outcome in OHCA patients in the 2015G compared to in the 2010G was 1.37 (95%CI 1.19-1.58, p<0.001). Conclusion: In the OHCA patients with non-shockable rhythm, the 2015 CPR guidelines were superior to the 2010 guidelines in terms of neurological benefits.