A head replacement-type cutting tool, comprising: a cutting head having a cutting part; and a holder which has a cylindrical leading end part and in which the cutting head is to be attached in a detachable manner to an attachment hole formed on the leading end part of the holder; wherein a fitting part is formed on the attachment hole so as to be in a tapered shape which gradually increases in the inner diameter (d) as close to an opening part of the attachment hole, a peripheral border of the opening part on a leading end face of the holder is perpendicular to a central axis of the attachment hole, at least the leading end face and the fitting part of the holder are made of a cemented carbide, a taper-shaped attachment part fitted to the fitting part is formed on the cutting head so as to protrude from a rear end face of the cutting part and to be coaxial with the central axis, the rear end face (11 C) of the cutting part is perpendicular to the central axis, at least the attachment part of the cutting head and the rear end face of the cutting part are made of the cemented carbide, characterised in that, the cutting head is attached to the holder in such a manner that a male thread part and a female thread part installed respectively are engaged with each other at the center of the central axis thereby the attachment part is fitted into the fitting part, and the leading end face is in contact with the rear end face, an inner diameter to outer diameter ratio d/D1 which is a ratio of an inner diameter d of the fitting part at the opening part of the attachment hole in a state before the attachment part is fitted into the fitting part in relation to an outer diameter D1 of the leading end part of the holder is in a range of 0.5 to 0.8, and an outer diameter expansion percentage (D2 -D1)/D1 x 100 (%) satisfies, 0.022 × d D 1 − 0.003 % ≦ D 2 − D 1 D 1 × 100 % ≦ 0.33 × d D 1 − 0.06 % wherein D1 is the outer diameter of the leading end part of the holder before the attachment part is fitted into the fitting part, and D2 is the outer diameter of the leading end part of the holder in a state where the attachment part is fitted into the fitting part and the cutting head is attached to the holder. a cutting head having a cutting part; and a holder which has a cylindrical leading end part and in which the cutting head is to be attached in a detachable manner to an attachment hole formed on the leading end part of the holder; wherein a fitting part is formed on the attachment hole so as to be in a tapered shape which gradually increases in the inner diameter (d) as close to an opening part of the attachment hole, a peripheral border of the opening part on a leading end face of the holder is perpendicular to a central axis of the attachment hole, at least the leading end face and the fitting part of the holder are made of a cemented carbide, a taper-shaped attachment part fitted to the fitting part is formed on the cutting head so as to protrude from a rear end face of the cutting part and to be coaxial with the central axis, the rear end face (11 C) of the cutting part is perpendicular to the central axis, at least the attachment part of the cutting head and the rear end face of the cutting part are made of the cemented carbide, characterised in that, the cutting head is attached to the holder in such a manner that a male thread part and a female thread part installed respectively are engaged with each other at the center of the central axis thereby the attachment part is fitted into the fitting part, and the leading end face is in contact with the rear end face, an inner diameter to outer diameter ratio d/D1 which is a ratio of an inner diameter d of the fitting part at the opening part of the attachment hole in a state before the attachment part is fitted into the fitting part in relation to an outer diameter D1 of the leading end part of the holder is in a range of 0.5 to 0.8, and an outer diameter expansion percentage (D2 -D1)/D1 x 100 (%) satisfies, 0.022 × d D 1 − 0.003 % ≦ D 2 − D 1 D 1 × 100 % ≦ 0.33 × d D 1 − 0.06 % wherein D1 is the outer diameter of the leading end part of the holder before the attachment part is fitted into the fitting part, and D2 is the outer diameter of the leading end part of the holder in a state where the attachment part is fitted into the fitting part and the cutting head is attached to the holder. wherein D1 is the outer diameter of the leading end part of the holder before the attachment part is fitted into the fitting part, and D2 is the outer diameter of the leading end part of the holder in a state where the attachment part is fitted into the fitting part and the cutting head is attached to the holder.