A head-mounted display device including a projection device and an optical waveguide is provided. The projection device has an optical pupil located on a second surface of the optical waveguide, and includes a light source, a first MEMS mirror element, a second MEMS mirror element, and a relay optical element group. The relay optical element group has a first axis equivalent focal length corresponding to a first parallel light beam and has a second axis equivalent focal length corresponding to a second parallel light beam. The first parallel light beam and the second parallel light beam travel along an optical axis of the relay optical element group, and a value of the first axis equivalent focal length is different from a value of the second axis equivalent focal length. The head-mounted display device may provide good image quality and a large field of view.