This method of producing 4-(((6S,9S)-1-(benzylcarbamoyl)-2,9-dimethyl-4,7-dioxo-8-(quinoline-8-ylmethyl) octahydro-1H-pyrazino[2,1-c][1,2,4]triazine-6-yl)methyl) phenyl dihydrogen phosphate involves a step for adding a reaction solution (1) comprising (6S,9S)-N-benzyl-6-(4-hydroxybenzyl)-2,9-dimethyl-4,7-dioxo-8-(quinoline-8-ylmethyl) octahydro-1H-pyrazino[2,1-c][1,2,4]triazine-1-carboxamide, triethylamine and a solvent to a reaction solution (2) comprising a phosphorylating agent and a solvent.