Abstract: A writing instrument internally houses a miniature electronic calculator which includes a key input section comprising a single key which is subjected to a rotational or translational movement to select one of a plurality of switches. When the key is depressed against a bias applied thereto while in a position in which it has selected one of the switches, the selected switch is controlled thereby to input an associated information into an arithmetic unit of the calculator. The instrument case or key is marked with indicia which designate the key positions for the convenience of selecting input informations. The key input section comprises an insulating substrate on which an array of a plurality of stationary contacts are formed along the direction of movement of the key, a flexible insulating film disposed in opposing relationship with the stationary contacts, and a movable contact deposited on the side of film which faces the stationary contacts so as to extend in the direction of movement of the key.