To provide a sophisticated security sheet blank, for an important document, which comprises a small number of individuals, featuring not only improved security through combining a conventional open work technique with a publicly known security knowhow, but also the requirement to rewrite only a spot where measures are taken to obtain an image formed by an open work from a transparent printing image.

This security sheet, for an important document, comprising a small number of individuals is made up of a support layer, an engraving layer which has a previously printed one image and is formed in one piece on the support layer, a back transparent matte layer which has a previously printed other different image and an appropriate surface roughness/a function to make the layer hardly chargeable and is integrally formed under the support layer, and a transparent printing layer which is integrally formed under the engraving layer and the back transparent matte layer. In addition, the transmission density of the support layer is not more than 0.15, and the transmission density of the support layer, the engraving layer and the back transparent matte layer is 0.15 or above and 1.00 or below. Besides, the surface glossiness of the engraving surface is not higher than 10.0%.