A lens adjusting device of a projector is provided. The lens adjusting device has a base, a frame, and at least one positioning structure. A lens is fixed on the frame. The positioning structure is utilized for movably setting the frame on the base. The positioning structure has a groove, a slider, and a positioning unit. The groove is disposed on the frame. The slider is disposed in the groove and has a guiding hole defined therein. The extending direction of the guiding hole is substantially perpendicular to the extending direction of the groove. The positioning unit is slidably disposed in the guiding hole and penetrates the guiding hole to set the frame and the slider on the base.