A residual chlorine measuring method includes the steps of: bringing a counter electrode 4, a working electrode 2 and a reference electrode 3 into contact with a sample solution containing residual chlorine; applying a voltage between the counter electrode 4 and the working electrode 2; and measuring a current value when applying the voltage to calculate a concentration of the residual chlorine. The working electrode 2 is an electrically conductive diamond electrode to which boron is doped. The reference electrode 3 is a silver/silver chloride electrode. A current value when setting the potential of the electrically conductive diamond electrode 2 to the silver/silver chloride electrode 3 to +0.5 V to +1.5 V is measured.