An antenna device includes a metal mechanism element, a ground plane, a feeding element, a grounding extension element, and a dielectric substrate. The metal mechanism element has a slot. The feeding element has a feeding point coupled to a signal source. The feeding element extends across the slot. The grounding extension element is coupled to the ground plane. A vertical projection of the grounding extension element at least partially overlaps the slot. An antenna structure is formed by the feeding element, the grounding extension element, and the slot of the metal mechanism element. The antenna structure is capable of covering a low-frequency band and a high-frequency band. The distance between the feeding point and one end of the slot is less than or equal to 0.1 wavelength of a central frequency of the low-frequency band.