A spindle head device for a machine tool comprising: - a spindle; and - a tool retaining mechanism detachably retaining a tool holder on said spindle; said tool retaining mechanism comprising: - a tool support member provided with a receiving recess detachably receiving a shank of a tool holder; - a tool clamp member disposed to be displaceable with respect to said tool support member between an operative position and an inoperative position, said tool clamp member, at said operative position, clamping the shank of the tool holder and fixedly holding the shank in said receiving recess of said tool support member, and at said inoperative position, releasing the shank and allowing the tool holder to be attached and detached to and from said receiving recess; and - a mounting member detachably mounting said tool support member and said tool clamp member to said spindle, - wherein said spindle is provided with a spindle-through-hole penetrating in an axial direction; and - wherein said tool support member is provided with a member-through-hole penetrating in an axial direction and communicating with said receiving recess, characterized in that - said tool support member is dimensioned so as to be contained in said spindle-through-hole of said spindle over an entire axial length of said spindle-through-hole; and - said tool clamp member comprises a movable clamping element contained in said member-through-hole of said tool support member to be displaceable in a radial direction, and an actuating element contained in said member-through-hole of said tool support member to be displaceable in an axial direction, said actuating element displacing said movable clamping element in the radial direction by an axial displacement of said actuating element. - a spindle; and - a tool retaining mechanism detachably retaining a tool holder on said spindle; said tool retaining mechanism comprising: - a tool support member provided with a receiving recess detachably receiving a shank of a tool holder; - a tool clamp member disposed to be displaceable with respect to said tool support member between an operative position and an inoperative position, said tool clamp member, at said operative position, clamping the shank of the tool holder and fixedly holding the shank in said receiving recess of said tool support member, and at said inoperative position, releasing the shank and allowing the tool holder to be attached and detached to and from said receiving recess; and - a mounting member detachably mounting said tool support member and said tool clamp member to said spindle, - wherein said spindle is provided with a spindle-through-hole penetrating in an axial direction; and - wherein said tool support member is provided with a member-through-hole penetrating in an axial direction and communicating with said receiving recess, characterized in that - said tool support member is dimensioned so as to be contained in said spindle-through-hole of said spindle over an entire axial length of said spindle-through-hole; and - said tool clamp member comprises a movable clamping element contained in said member-through-hole of said tool support member to be displaceable in a radial direction, and an actuating element contained in said member-through-hole of said tool support member to be displaceable in an axial direction, said actuating element displacing said movable clamping element in the radial direction by an axial displacement of said actuating element. - a tool support member provided with a receiving recess detachably receiving a shank of a tool holder; - a tool clamp member disposed to be displaceable with respect to said tool support member between an operative position and an inoperative position, said tool clamp member, at said operative position, clamping the shank of the tool holder and fixedly holding the shank in said receiving recess of said tool support member, and at said inoperative position, releasing the shank and allowing the tool holder to be attached and detached to and from said receiving recess; and - a mounting member detachably mounting said tool support member and said tool clamp member to said spindle, - wherein said spindle is provided with a spindle-through-hole penetrating in an axial direction; and - wherein said tool support member is provided with a member-through-hole penetrating in an axial direction and communicating with said receiving recess, characterized in that - said tool support member is dimensioned so as to be contained in said spindle-through-hole of said spindle over an entire axial length of said spindle-through-hole; and - said tool clamp member comprises a movable clamping element contained in said member-through-hole of said tool support member to be displaceable in a radial direction, and an actuating element contained in said member-through-hole of said tool support member to be displaceable in an axial direction, said actuating element displacing said movable clamping element in the radial direction by an axial displacement of said actuating element.