To provide an expansion/contraction changing mechanism for a honing stick in which form precision of a work to a ground surface of a work can be improved, and in which finishing efficiency can be improved.

In an expansion/contraction changing mechanism 10 for a honing stick 11, an expansion/contraction rod 21 is rotated around its axis, so two taper cones 20 threaded with a normal thread part 22 and an inverse thread part 23 in the expansion/contraction rod 21 to face each other get closer to/ apart from each other in an axial direction of the expansion/contraction rod 21. The honing stick 11 energized to taper surfaces of the two taper cones 20 is expanded/contracted in a radial direction of the expansion/contraction rod 21. At this time, force applied to the expansion/contraction rod 21 is balanced in the expansion/contraction rod 21, so thrust load is not generated in the expansion/contraction rod 21. By providing a pin 25 in a guide groove of the taper cone 20, the taper cone 20 can be prevented from integrally rotating with the expansion/contraction rod 21.