A drill comprising: a drill main body; a plurality of cutting edges (4a, 4b) which are formed on a tip side of the drill main body from an outer periphery of the drill main body to a chisel edge provided near to a rotational axis; thinnings (12a, 12b) which are each formed between the cutting edges; flutes (15a, 15b) which are each formed from a posterior end of each of the thinnings (12a, 12b) in a direction of the rotational axis toward a posterior end side of the drill main body; first margins (8a, 8b) which are each formed along an outer peripheral end of the drill main body on a back side of each of the flutes (15a, 15b) in a rotational direction; second margins (9a, 9b) which are each formed along the outer peripheral end of the drill main body on a forward side of each of the flutes (15a, 15b) in the rotational direction; second faces (5a, 5b) which are each formed in an approximately band shape and are each formed along each of the cutting edges on a back side of the cutting edge in the rotational direction; third faces (6a, 6b) which are each formed to be continuous with a back side of each of the second faces (5a, 5b) in the rotational direction; and fourth faces (7a, 7b) which are each formed to be continuous with a back side of each of the third faces in the rotational direction; wherein a tip side of each of the thinnings (12a, 12b) is continuous with the back side of each of the third faces in the rotational direction, a posterior end side of each of the thinnings (12a, 12b) is continuous with a back side of each of the fourth faces (7a, 7b) in the rotational direction, characterised in that the cutting edges (4a, 4b) extend in a convex-arc shape from the outer periphery of the tip portion to the chisel edge, and in a case where the drill main body is viewed from the tip side thereof, each of boundaries between the thinnings (12a, 12b) and the third faces forms an angle (ε) of 35° to 55° with each of boundaries between the thinnings (12a, 12b) and the fourth faces (7a, 7b). a drill main body; a plurality of cutting edges (4a, 4b) which are formed on a tip side of the drill main body from an outer periphery of the drill main body to a chisel edge provided near to a rotational axis; thinnings (12a, 12b) which are each formed between the cutting edges; flutes (15a, 15b) which are each formed from a posterior end of each of the thinnings (12a, 12b) in a direction of the rotational axis toward a posterior end side of the drill main body; first margins (8a, 8b) which are each formed along an outer peripheral end of the drill main body on a back side of each of the flutes (15a, 15b) in a rotational direction; second margins (9a, 9b) which are each formed along the outer peripheral end of the drill main body on a forward side of each of the flutes (15a, 15b) in the rotational direction; second faces (5a, 5b) which are each formed in an approximately band shape and are each formed along each of the cutting edges on a back side of the cutting edge in the rotational direction; third faces (6a, 6b) which are each formed to be continuous with a back side of each of the second faces (5a, 5b) in the rotational direction; and fourth faces (7a, 7b) which are each formed to be continuous with a back side of each of the third faces in the rotational direction; wherein a tip side of each of the thinnings (12a, 12b) is continuous with the back side of each of the third faces in the rotational direction, a posterior end side of each of the thinnings (12a, 12b) is continuous with a back side of each of the fourth faces (7a, 7b) in the rotational direction, characterised in that the cutting edges (4a, 4b) extend in a convex-arc shape from the outer periphery of the tip portion to the chisel edge, and in a case where the drill main body is viewed from the tip side thereof, each of boundaries between the thinnings (12a, 12b) and the third faces forms an angle (ε) of 35° to 55° with each of boundaries between the thinnings (12a, 12b) and the fourth faces (7a, 7b).