Upon request for retrofitting, workers bring a gadget and a marking tool and other tools to the user's home; the workers attach the marking tool to a keyboard, mark locators on a key bed by using the marking tool, form locating recesses therein, and bore reference seal holes by using a template, counter holes of which are aligned with the locating recesses; subsequently, the workers relate the gadget with the reference seal holes, bore through-holes in the key bed at target spots exactly below the black and white keys under the guide of the reference seal holes, and assemble a key drive unit with the key bed so that plungers pass through the through-holes; since the reference seal holes have been related with the keyboard, the gadget sequentially positions a drill at the target spots also related to the keyboard.