A drill bit insert to be attached to an end portion of a drill bit to perform drilling, the drill bit insert comprising: an insert body; and a hard surface layer coated on the insert body and made of a polycrystalline diamond, which is harder than the insert body, wherein the insert body comprises: a rear end portion forming a columnar shape or a disk-like shape centered on a center line of the insert; an intermediate portion arranged at a tip side of the rear end portion in a direction of the center line of the insert, and having an outer diameter smaller than that of the rear end portion; and an end portion arranged at a tip side of the intermediate portion in the direction of the center line of the insert, and having an outer diameter from the center line of the insert gradually decreasing toward the tip side, wherein the hard surface layer is coated on the insert body from a surface of the end portion of the insert body to an outer periphery of the intermediate portion, an outer diameter of the hard surface layer on the intermediate portion is equal to that of the rear end portion of the insert body, the intermediate portion forms a columnar shape or a disk-like shape, centered on the center line of the insert and having the outer diameter smaller than that of the rear end portion, characterized in that the thickness of the hard surface layer of the intermediate portion in a radial direction perpendicular to the center line of the insert is constant in a direction over the center line of the insert, and the thickness of the hard surface layer is thick at the tip of the end portion on the center line of the insert, and the thickness decreases from this tip toward the outer peripheral side of the end portion. an insert body; and a hard surface layer coated on the insert body and made of a polycrystalline diamond, which is harder than the insert body, wherein the insert body comprises: a rear end portion forming a columnar shape or a disk-like shape centered on a center line of the insert; an intermediate portion arranged at a tip side of the rear end portion in a direction of the center line of the insert, and having an outer diameter smaller than that of the rear end portion; and an end portion arranged at a tip side of the intermediate portion in the direction of the center line of the insert, and having an outer diameter from the center line of the insert gradually decreasing toward the tip side, wherein the hard surface layer is coated on the insert body from a surface of the end portion of the insert body to an outer periphery of the intermediate portion, an outer diameter of the hard surface layer on the intermediate portion is equal to that of the rear end portion of the insert body, the intermediate portion forms a columnar shape or a disk-like shape, centered on the center line of the insert and having the outer diameter smaller than that of the rear end portion, characterized in that the thickness of the hard surface layer of the intermediate portion in a radial direction perpendicular to the center line of the insert is constant in a direction over the center line of the insert, and the thickness of the hard surface layer is thick at the tip of the end portion on the center line of the insert, and the thickness decreases from this tip toward the outer peripheral side of the end portion.