Provided are a cap capable of properly injecting contents stored in a container body into an apparatus, a container including the cap, a water softening device, and a connection mechanism. A cap for connecting an apparatus that is configured to be attachable to a container body storing contents and includes an injection part formed with an injection port through which the contents are injected, and the container body, the cap including a cylindrical cap body 21 that forms a flow path through which the contents flow from the container body, a plug part 22 that opens and closes the flow path, Wherein the plug portion is configured to engage with the cap body to close the flow path and open the flow path when the engagement is released, The plug is configured to engage and disengage the injection portion with the container body coupled to the instrument. Fig. 12

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