The World Health Organization (WHO) recommends antimicrobial use density (AUD) as an indicator for evaluating the amount of antimicrobials used, an index that is now widely employed in many facilities. Defined daily doses (DDD) set by WHO are used for calculating AUD. However, discrepancies have been noted between other countries and Japan in the standard dosage of antimicrobials, which may cause a problem evaluating antimicrobial use with the DDD. Therefore, in this study, we calculated AUD (modified antimicrobial use density: mAUD) with the DDD (modified defined daily dose: mDDD) of our hospital for the carbapenem antimicrobial meropenem (MEPM), mAUD, and resistance rate of Pseudomonas aeruginosa. From 2010 through fiscal year 2016 (ending in March), AUD was 5.9±1.4, 7.0±2.9, 8.2±2.3, 6.8±2.1, 7.3±2.2, 7.0±2.1, and 8.0±3.0 and mAUD was 11.7±2.7, 12.0±4.9, 11.3±3.1, 11.0± 3.4, 11.4±3.5, 11.5±3.5, and 11.2±4.2, respectively. The corresponding resistance rate of P. aeruginosa was 35.1%, 37.9%, 10.0%, 6.0%, 22.6%, 10.6%, and 10.0%. A significant positive correlation was found between mAUD and the resistance rate of Pseudomonas aeruginosa (P < 0.01, r = 0.88). Our results confirm that the mAUD is an effective index for controlling resistance of P. aeruginosa.