1. A transmission type head-mounted display device comprising: a sound sensor configured to acquire sound from an outside environment of the head-mounted display device; an image display capable of displaying an image and capable of transmitting an outside scene; a memory storing: (i) a plurality of execution function images corresponding to respective functions executable by the head-mounted display device, and (ii) a plurality of specific sound images corresponding to respective sounds; a processor or integrated circuit programmed to: cause the image display to display a combined image in a first form, the combined image in the first form being obtained through combining one of the plurality of execution function images and one of the plurality of specific sound images; identify a specific sound corresponding to the specific sound image being displayed as a part of the combined image on the image display from sound data acquired by the sound sensor; in response to the identified specific sound, execute the function corresponding to the execution function image combined with the specific sound image displayed as a part of the combined image; and when a predetermined time elapses without identifying the specific sound after the combined image in the first form is displayed, cause the image display to display the combined image in a second form such that the execution function image at least partially disappears and the specific sound images is kept displayed.