An adapter, a light source device employing the same, and an illuminating device employing the same are provided. The adapter includes a main body, an ejector pin movably connected with the main body, and an operating member disposed between the ejector pin and the main body. The main body includes: a power supply mounting part, a light source mounting part, and a conducting wire extended from the power supply mounting part to the light source mounting part. The ejector pin is operationally moved to a blocking position for preventing the power supply mounting part from being matched with the power supply module and a release position for releasing the power supply mounting part to be matched with the power supply module. The operating member is operationally moved on the main body to an ‘on’ position and an ‘off’ position.