A reamer, which has a center axis and includes a single cutting edge, provided for a cutting part that is arranged at a leading end side, comprising: at least three guide pads (28, 30, 32) arranged at a distance from each other around the cutting part; wherein each of the at least three guide pads extends substantially parallel to the center axis; of the at least three guide pads, a first guide pad is formed so as to project to the side almost opposite that where the single cutting edge is formed; a second guide pad of the at least three guide pads is arranged in one area of two areas, along a peripheral face of the cutting part, that are located between the first guide pad and the single cutting edge and that are directed toward sides opposite each other; and a third guide pad of the at least three guide pads is arranged in the other area, different from the one area, of the two areas that are directed toward the sides opposite each other; wherein, when a direction in which the reamer is to be relatively rotated about the center axis relative to a workpiece is defined as a rotational direction, the second guide pad is positioned to the front, in the rotational direction, by employing the cutting edge as a reference, and the third guide pad is positioned to the rear in the rotational direction; wherein, the first guide pad includes one peripheral face or two guide pad portions (29a, 29b) forming the peripheral face of the first guide pad, wherein the peripheral face of the first guide pad, the peripheral face of the second guide pad, and the peripheral face of the third guide pad extends along a cylindrical face that is defined as passing the corner of the cutting edge about the center axis; and characterised in that, a circumferential length of the peripheral face of the first guide pad is set within a range of 8% to 15% of a circumferential length of the cylindrical face, a circumferential length of the peripheral face of the second guide pad is set within a range of 0.8% to 6% of a circumferential length of the cylindrical face, and a circumferential length of the peripheral face of the third guide pad is set within a range of 0.8% to 6% of a circumferential length of the cylindrical face. at least three guide pads (28, 30, 32) arranged at a distance from each other around the cutting part; wherein each of the at least three guide pads extends substantially parallel to the center axis; of the at least three guide pads, a first guide pad is formed so as to project to the side almost opposite that where the single cutting edge is formed; a second guide pad of the at least three guide pads is arranged in one area of two areas, along a peripheral face of the cutting part, that are located between the first guide pad and the single cutting edge and that are directed toward sides opposite each other; and a third guide pad of the at least three guide pads is arranged in the other area, different from the one area, of the two areas that are directed toward the sides opposite each other; wherein, when a direction in which the reamer is to be relatively rotated about the center axis relative to a workpiece is defined as a rotational direction, the second guide pad is positioned to the front, in the rotational direction, by employing the cutting edge as a reference, and the third guide pad is positioned to the rear in the rotational direction; wherein, the first guide pad includes one peripheral face or two guide pad portions (29a, 29b) forming the peripheral face of the first guide pad, wherein the peripheral face of the first guide pad, the peripheral face of the second guide pad, and the peripheral face of the third guide pad extends along a cylindrical face that is defined as passing the corner of the cutting edge about the center axis; and characterised in that, a circumferential length of the peripheral face of the first guide pad is set within a range of 8% to 15% of a circumferential length of the cylindrical face, a circumferential length of the peripheral face of the second guide pad is set within a range of 0.8% to 6% of a circumferential length of the cylindrical face, and a circumferential length of the peripheral face of the third guide pad is set within a range of 0.8% to 6% of a circumferential length of the cylindrical face.