An external lens device is mounted in front of a lens of an image pickup device. The external lens device includes a fixer and an external lens. The fixer includes a fixing slot and an alignment slice. The alignment slice includes a first reference mark and a second reference mark. The first reference mark is enclosed by the second reference mark. The first reference mark is arranged in the middle portion of the second reference mark. After the fixer is placed in front of the image pickup device, an outer portion of the second reference mark of the alignment slice is parallel with the lens fastening part of the lens of the image pickup device, the first reference mark is aligned with a center of the lens of the image pickup device, and the second reference mark is concentric with a lens fastening part of the lens.