Docking unit and electronic apparatus system

To provide a docking device capable of securing heat dissipation performance of a portable information apparatus while extending the function of an electronic apparatus in which an extension unit is mounted on the portable information apparatus, and an electronic apparatus system having the docking device. A docking unit that has a docking portion capable of docking an electronic apparatus in which a mounting surface of an extension unit is detachably mounted on a back surface of the portable information apparatus, a holding portion holding the portable information apparatus at a predetermined position in a docking direction when the electronic apparatus is docked with the docking portion, and a forcibly moving portion forcibly moving the extension unit in the docking direction when the electronic apparatus is docked with the docking portion.