A method of producing, at a high conversion rate and with good reproducibility, a 3‒[(S)‒7‒bromo‒2‒(2‒oxo‒propylamino)‒5‒pyridine‒2‒yl‒3H‒1,4‒benzodiazepine‒3‒yl]methyl propionate through the oxidization of a 3‒[(S)‒7‒bromo‒2‒(2‒hydroxy‒propylamino)‒5‒pyridine‒2‒yl‒3H‒benzo[e][1,4]diazepine‒3‒yl]methyl propionate by means of an oxidation catalyst is provided by regulating the ammonium ion content of the 3‒[(S)‒7‒bromo‒2‒(2‒hydroxy‒propylamino)‒5‒pyridine‒2‒yl‒3H‒benzo[e][1,4]diazepine‒3‒yl]methyl propionate.
