Abstract: A low-cost input device suitable for a wearable computer is provided. An input method suitable for operating a wearable computer is provided. A wearable computer including the input device is provided. An input device 100 for outputting a control command ? to an external output device 40 includes a sensor 10 configured to be worn in or on a natural orifice of the body and sense a change in a shape of the natural orifice, and a controller 20 configured to control the external output device 40 based on a sensor signal ? from the sensor 10. Here, the change in the shape of the natural orifice is caused by a predetermined action which is performed by a user wearing the sensor 10 with the intention of operating the external output device 40.