A one-touch fitting type adapter and a filter assembly are provided in which the filter assembly used for machinery, such as a bidet or a refrigerator, is easily assembled with or disassembled from the adapter through a one-touch fitting manner such that the filter assembly is easily adapted for the machinery. A multiple adapter capable of coupling a plurality of adapters in series is provided such that a plurality of filter assemblies used for a water purifier, a water ionizer or a functional water maker are detachably engaged with the multiple adapter through a one-touch fitting manner.