A shim attaching/detaching tool for a tappet of an internal combustion engine includes as essential components an air supplying tube, an air blowing tube and a nozzle formed at an opening portion of the air blowing tube at the foremost end of the same. When compressed air is blown from the nozzle toward the shim while the air supplying tube is inclined at an angle of about 45 degrees and the nozzle is oriented toward the fitting portion of the shim fitted into a circular cavity, the shim is thrusted against the opposite side of the cavity and the compressed air is then introduced into the space below the shim through the gap appearing around the fitting portion of the shim, whereby the shim is floated up away from the cavity so as to enable it to be easily exchanged with another shim having a different thickness.