Introduction: Mastitis is the inflammation of the mammary gland that mainly occurs during the lactation period and requires antibiotic treatment with little evidence for its efficacy. We investigated the effect of National Action Plan for antimicrobial resistance aiming for appropriate antimicrobial stewardship on broad-spectrum antibiotics administration for mastitis despite the lack of a disease-specific antimicrobial agents manual. Methods: Using a large nationwide database, an interrupted time-series analysis was performed using data from 34,340 female patients who received antibiotics for mastitis between April 2012 and March 2020. This study compared the trend of outcomes before and after the publication date of the National Action Plan (April 2016). The outcomes were the proportion of broad-spectrum and first-choice narrow-spectrum antibiotic administration and surgical drainage within 30 days after the administration. Results: Broad-spectrum antibiotics were administered in 70% of the cases before and 67% of the cases after the National Action Plan publication date. The trend of broad-spectrum antibiotics administration significantly changed at the publication (-2,6% [95% confidence interval, -3.9% to -1.3%], P<0.001) and the administration decreased after the publication (1.9% annual decrease, Ptrend<0.001). The trend of first-choice antibiotics administration also changed at the publication (1.3% [0.1% to 2.4%], P=0.028) and the administration increased after the publication (1.3% annual increase, Ptrend<0.001). The occurrence of surgical drainage was stable during the study period. Conclusion: Despite the lack of a disease-specific antimicrobial manual, the publication of the National Action Plan improved antimicrobial stewardship for mastitis without any impact on a surgical treatment course.