To provide a washing apparatus of a chemical feed system for semiconductor treatment wherein improved energy efficiencies, washing efficiencies, and improved durability can be provided, and its washing method.

The washing apparatus 30 for washing the interior of a flow path of a channel constituting body 12 has an airtight casing 32 detachably connected to the body 12. A washing liquid feed port 52 is provided in the casing 32 to feed washing liquid into the flow path of the body 12 through the casing 32. An ultrasonic wave generator 42 is mounted to the casing 32 to transfer ultrasonic waves to the casing 32 while a conversion horn 46 is connected to the generator 42 to convert the ultrasonic waves to vibration. The horn 46 extends from within the casing 32 into the flow path of the body 12 approximately coaxially therewith.