An optical device and a wearable image display device are provided with a light guiding plate, a plurality of half mirrors, and an incident angle adjustment member. The optical member that causes the video light to be incident as a virtual image on a person's eye and be displayed as a display image, and has a reflection characteristic that a part of the video light is reflected on a back surface thereof. The incident angle adjustment member is an optical member configured to adjust the incident angle of the video light according to the reflection characteristic of the half mirror to form video light that does not satisfy a total reflection condition in another part of the video light, to balance the brightness of the display image.