A mouse and a back cover of a holding device adjustable for left and right hand shapes are provided. The mouse includes: a key module that includes at least one button unit, a circuit base that includes a bottom casing and a control module, and a housing module fixed to the circuit base. The control module is electrically connected to the at least one button unit. The housing module includes a movable housing and a fixing assembly. The movable housing is operable to move left or right relative to the circuit base and switch between left-side and right-side positions. The fixing assembly is operable to fix the movable housing, which is located at the left-side or the right-side position, with the circuit base. When the movable housing is located at the left-side or the right-side position, the mouse is adapted for a right-handed or a left-handed user, respectively.