An optical device including a first substrate, a light source, a second substrate, an image capturing device, a lens module and a lens holder is provided. The light source outputs a first light beam. The second substrate includes a first surface and a second surface opposite to the first surface and closer to the first substrate. A scattered light beam which is generated by the first light beam entering an object touching the first surface of the second substrate and scattered in the object is a second light beam. The image capturing device receives a third light beam. The third light beam is the second light beam normally incident to the second surface and transmitted to the image capturing device. The lens module focuses the third light beam to be captured by the image capturing device. The lens holder is located between the light source and the image capturing device.