A head-mounted display device including a front piece, a plurality of headbands, and an adjustment mechanism is provided. The front piece is provided with or adapted to house a display device. An end of each of the headbands is connected to the front piece. The adjustment mechanism includes a base, a turntable, and a knob. The turntable and the knob are pivoted to the base on an axis of rotation. The turntable has a plurality of satellite points. Each of the satellite points is spaced apart from the axis of rotation, and another end of each of the headbands is connected to the corresponding satellite point. By turning the knob to rotate the turntable, the headbands are wound around the periphery of the satellite points to simultaneously reduce the length of use of the headbands.