PROBLEM TO BE SOLVED: To provide a reagent and method for measuring urea nitrogen, capable of accurately measuring urea nitrogen in a specimen by preventing ammonia from making a positive error to a measured value arise and by completely terminating a coupled reaction due to an isocitrate dehydrogenase. SOLUTION: This reagent for measuring urea nitrogen includes trans-1,2- diaminocyclohexane-N,N,N',N'-tetraacetic acid (CyDTA) or its salt as a reaction stopper for a coupled reaction due to an isocitrate dehydrogenase. This method for measuring urea nitrogen adopts trans-1,2-diaminocyclohexane-N,N,N',N'- tetraacetic acid (CyDTA) or its salt as a reaction stopper for a coupled reaction due to an isocitrate dehydrogenase.
