BACKGROUND: It is recommended globally that shocks by automated external defibrillators (AEDs) should be delivered immediately when a shockable out-of-hospital cardiac arrest (OHCA) occurs. However, the actual time-interval from collapse to first shock by public-access AED and its impact on subsequent outcome has not been extensively investigated in real-world settings. METHODS: OHCA data from 2013 to 2015 were obtained from the All-Japan Utstein Registry. Bystander-witnessed OHCA patients with shockable rhythm who were shocked by public-access AED in public locations were included. The primary endpoint was 1-month survival with favourable neurological outcome, and the association between time-interval from collapse to first shock by public-access AED and subsequent outcome was assessed. RESULTS: During the study period, 28% (2282/8126) of bystander-witnessed OHCA cases with shockable rhythm were shocked by public-access AED in public locations. The proportion of OHCA patients who were shocked by public-access AED within 5 min from collapse was 58% (1323/2282). Among these patients, the proportion of 1-month survival with favourable neurological outcome was 62% (815/1317). The proportion significantly decreased with increased time from collapse to shock by public-access AED (48% for 6-10 min, 38% for 11-15 min, 30% for 16-20 min, and 7% for 21-25 min; p-for-trend <0.001), and no patient survived if shock delivery occurred more than 26 min after OHCA. CONCLUSION: In Japan, earlier shock by public-access AED led to better outcome after bystander-witnessed OHCA with shockable rhythm in public locations. However, the proportion of OHCA patients who received early shock was still low in public locations.