A method of purifying a cleaning agent is provided. The method includes heating a first mixed solution including an etching agent, a first cleaning agent, and a second cleaning agent at or below a first temperature and distilling the etching agent and the first cleaning agent and removing the second cleaning agent. The method includes condensing or compressing the etching agent and the first cleaning agent forming a second mixed solution including the etching agent and the first cleaning agent. The method includes heating the second mixed solution at a temperature lower than a second temperature, redistilling the etching agent and extracting the first cleaning agent. The second temperature is lower than the first temperature.