To provide an adapter which is good for reducing cost at production, prevents erroneous usage by eliminating a push button used in a conventional can, is serviceable for certain and deliberate use of contents while being usable repetitiously.

An adapter comprises a flow-out passage 1, a valve operating device 2 facing the flow-out passage 1, a residual quantity meter 3 communicating with the flow-out passage 1, and a attachment part 4 which is attachable to and detachable from an aerosol can 5. The flow-out passage 1 receives a valve 17 of the aerosol can 5 at its one end and is formed at the other end into a nozzle which is opened and closed by operation of the valve operating device 2. The residual quantity meter 3 detects the residual quantity o fluid in the aerosol can by detecting the pressure of fluid flowing out to the flow-out passage 1. The attachment part 4 is for attaching the adapter to and detaching the adapter from the aerosol can and formed in a structure adaptable to the shape of a part 9 for attachment at the aerosol can. A male- and -female screwing type is a generally accepted structure, but a click mechanism and the like may be adopted.