A porous structure with high uniformity is provided even when evaluated at a high resolution (high evaluation standard) of several or several ten nm or less. By applying this porous structure to the manufacture of an SOI substrate, an SOI substrate which has an SOI layer with a small number of defects is provided. In a region at a depth of 5 to 10 nm from the surface of a porous Si layer, values of parameters such as porosity and the like which represent a porous structure are uniformed. The manufacture of an SOI substrate using this porous Si layer reduces recessed defects in an SOI layer.