Newly found single-wall carbon nanohorns (SWNH) are synthesized by a CO2 laser vaporization of a pure carbon target in an Ar gas. Purification method using centrifugal separation and structural features of aggregated form of SWNHs (SWNH aggregate) are studied in this Letter. Although the X-ray diffraction profile taken for highly purified SWNH aggregates does not show any sharp 002 diffraction corresponding to that of the ordinary graphite, a huge broad diffraction comes to be observed. This huge broad diffraction is well explained by the double layer diffraction model associated with two graphene sheets apart from 0.40 nm.