PROBLEM TO BE SOLVED: To provide a washing method of a milking system capable of appropriately reducing energy consumption by adjusting and controlling the temperature of washing liquid appropriately and simply.SOLUTION: There is provided a method of washing a part to be washed of a milking system by circulating washing liquid with temperature, which is adjusted by mixing high-temperature water from a hot water supply source (water heater 22) and low-temperature water from a water supply source, from a washing tank 18. The length of supply time low-temperature water to be mixed with high-temperature water is set to a plurality of water supply steps. The temperature of drainage water of washing liquid, which is returned to the washing tank 18 after washing the part to be washed and discharged from the washing tank 18, is measured. Then, control is performed to change a water supply step by one step so as to make the length of supply time of low-temperature water longer than a previous water supply step when the temperature of the drainage water is a predetermined temperature or higher, and change the water supply step by one step so as to make the length of supply time of low-temperature water shorter than a previous water supply step when the temperature of the drainage water is a predetermined temperature or lower.
