Disclosed herein is a method of automatically cleaning boiler pipes. The provision of a pipe-cleaning tank on one side of a casing to store a cleaning solution therein and a tank supply valve in the pipe-cleaning tank allows the cleaning solution stored in the pipe-cleaning tank to automatically flow into a heat exchanger and various pipes inside the casing by means of only a simple operation of manipulating the tank supply valve, thereby making it easy to eliminate scale that has accumulated in the heat exchanger and the pipes.