Abstract: A spindle attachment device attached, for use, to a distal end of a spindle of a machine tool is provided. The spindle attachment device (11) includes a hollow casing (13) having an internal space; an input shaft (17) rotatably supported inside the casing; an output shaft (19) rotatably supported inside the casing; a coupling member (15) connected to the input shaft in order to join the input shaft to the spindle of the machine tool; a gear train (21) disposed between the input shaft and the output shaft in order to increase a rotation speed of the input shaft for a transmission to the output shaft; and a tap holder (25, 27) for mounting a tapping tool on the output shaft. The spindle attachment device increases the rotation speed of the spindle and transfers it to the tapping tool, and thus makes it possible to perform a tapping process on a workpiece at high speed, while maintaining the rotation speed of the spindle at a low level.