A handle substrate of a composite substrate for a semiconductor; said handle substrate comprising an insulating polycrystalline material, wherein said handle substrate has a surface having a microscopic central line average surface roughness Ra of 5nm or smaller; wherein recesses are present on said surface on intergranular boundaries between some crystal grains of said polycrystalline material or, if a size of some crystal grains is 15 μm or more, inside the some crystal grains of said polycrystalline material at the surface, the crystal grains are randomly distributed; wherein a density of said recesses having a diameter of 0.5μm or larger is 50 counts or larger and 4500 counts or smaller per 1cm 2 of said surface, an average of depths of said recesses is 0.1μm or larger and 0.8μm or smaller, an average of a diameter of said recesses is 1μm or larger and 5μm or smaller, the density is determined by means of a differential interference contrast microscope (magnitude of 500) and illumination onto the side face of the substrate by a fiber light, the observed visual field was made 500μm×500μm, the substrate plane was divided into 9 sites of 3×3 at a constant interval, and the observation was made in one visual field for each site to obtain results, whose average was calculated to obtain the density, a distribution of the depths of the recesses is measured by means of an interferometer with an objective×50, a zoom×1, and a visual field for the measurement of 140μm×110μm, a maximum value of the depths of the observed recess is defined as the depth of the recess, wherein a number of visual fields are observed such that a total number of the measured recesses is 100 or larger, and the average depth is obtained from the distribution of the depths, the diameter of each of the determined recesses used for determining the average of the diameter is determined as a diameter of a circle having the minimum area and capable of including the whole of the respective determined recess, the diameters are measured by means of a laser microscope at a magnitude of 1000, the visual field to be measured is made 200μm×200μm, wherein a number of visual fields are observed such that a total number of the determined recesses is 100 or larger. a density of said recesses having a diameter of 0.5μm or larger is 50 counts or larger and 4500 counts or smaller per 1cm 2 of said surface, an average of depths of said recesses is 0.1μm or larger and 0.8μm or smaller, an average of a diameter of said recesses is 1μm or larger and 5μm or smaller, the density is determined by means of a differential interference contrast microscope (magnitude of 500) and illumination onto the side face of the substrate by a fiber light, the observed visual field was made 500μm×500μm, the substrate plane was divided into 9 sites of 3×3 at a constant interval, and the observation was made in one visual field for each site to obtain results, whose average was calculated to obtain the density, a distribution of the depths of the recesses is measured by means of an interferometer with an objective×50, a zoom×1, and a visual field for the measurement of 140μm×110μm, a maximum value of the depths of the observed recess is defined as the depth of the recess, wherein a number of visual fields are observed such that a total number of the measured recesses is 100 or larger, and the average depth is obtained from the distribution of the depths, the diameter of each of the determined recesses used for determining the average of the diameter is determined as a diameter of a circle having the minimum area and capable of including the whole of the respective determined recess, the diameters are measured by means of a laser microscope at a magnitude of 1000, the visual field to be measured is made 200μm×200μm, wherein a number of visual fields are observed such that a total number of the determined recesses is 100 or larger.