To provide grinding wheel unit and grinding wheel changing device for a processing machine for automatically changing a grinding wheel.

In a grinding wheel fastening bolt 44, a screw part screwed into a screw hole formed on a recessed surface grinding wheel spindle, and a rotating direction constraining axis direction non-constraining part to be rotated together with a rotary shaft 27 and free for the rotary shaft 27 in the axis direction are formed. The grinding wheel fastening bolt 44 is rotated in forward direction and reverse direction to fix a grinding wheel flange to the grinding wheel spindle and to release the grinding wheel flange from the grinding wheel spindle. The grinding wheel spindle and the grinding wheel 29 are easily attached/detached by axial direction propulsion and rotating direction propulsion of the grinding wheel fastening bolt 44 sharing a center axial line.