1. A medical diagnosis apparatus configured to obtain a signal for generating an image of an observation target and configured to cause an external display to display an observation image based on the obtained signal to perform diagnosis, the medical diagnosis apparatus comprising: a detection unit including a sensor that receives light reflected from a finger of a practitioner and converts the received light to a detection signal, the detection unit being configured to produce a signal pattern in accordance with the detection signal, to detect a shape of the finger of the practitioner by matching the signal pattern with previously acquired signal patterns, and to detect a spatial position of the finger of the practitioner in accordance with the detection signal; and a processor comprising hardware, the processor being configured to: select one or more commands executable by the medical diagnosis apparatus in accordance with the shape and the spatial position of the finger of the practitioner detected by the detection unit; generate a command image corresponding to the selected one or more commands and generate a finger shape image indicating the shape of the finger of the practitioner; and generate a display image using the generated command image, the finger shape image and the observation image.