a cylindrical base provided with a plurality of radially thickened cutting portion forming portions spaced apart from one another in a circumferential direction, the cutting portion forming portions formed only adjacent a tip end portion in an axial direction of the base, the thickened cutting portion forming portions protruding radially outward from an outer peripheral surface of the base, each cutting portion forming portion having a front surface and a back surface;

an attaching portion that is provided on a base end side of the base to be attachable to a rotation drive tool;

a plurality of cutting portions provided at a tip end of the base to be spaced apart from each other in a circumferential direction of the base in such a manner that outer peripheries of the cutting portions protrude radially outward relative to an outer periphery of the base; and

discharge grooves each of which is provided between the back surface of each cutting portion forming portion and the front surface of the next adjacent cutting portion forming portion in a rotationally rearward direction, the discharge grooves being provided only in the tip end portion of the cylindrical base and being configured to discharge chips cut by the cutting portions toward the base end of the base;

wherein each of the cutting portions is constructed of an integral block separate from the base and securable to the base, each cutting portion having at least three cutting blades including a first cutting blade, a second cutting blade, and a third cutting blade which are arranged radially adjacent each other with the cutting portions attached to the base, the first cutting blade being located radially inward relative to the second and third cutting blades and the second cutting blade is located between the first and third cutting blades in a radial direction of the base, wherein the first, second, and third cutting blades are arranged such that their circumferential positions deviate rearward in this order in a rotational direction of the core cutter;

wherein a wall of a rear end of the discharge groove provided to correspond to the third cutting blade is located at the same position or rearward in the rotational direction relative to a cutting surface of the third cutting blade; and

wherein a gallet for discharging chips cut by each cutting portion is provided adjacent each cutting portion to be located forward relative to each cutting portion in the rotational direction, the gallet being formed by a single curved surface, the single curved surface formed from part of a conical surface, the single curved surface formed by cutting an outer peripheral surface of a tip end portion of the base in such a manner that an inner end of the gallet is located radially inward relative to an outer end of the first cutting portion and protrudes to a lower position than an outer end of the gallet, and wherein a tip end of the gallet extends horizontally relative to a vertical axis of the cylindrical base.