An illuminating device includes an aluminum extrusion profile fixed on a back panel of a showcase, and a lamp device. A face panel of the showcase is secured to an end of the aluminum extrusion profile away from the back panel. In the middle of the aluminum extrusion profile defines a sliding slot extending horizontally. The face panel defines an elongated slot corresponding to the sliding slot. The lamp device includes a lamp and a conductive head. A conductive structure is secured to the sliding slot for allowing the conductive head to be plugged therein and conduct the conductive head. The conductive head can slide along the sliding slot to be located at any desired position to illuminate objects on the showcase. A ratchet connecting mechanism is arranged on the sliding slot and configured to mount and remove the conductive head.