A jig is used in an assemblage between key sensors (11/ 15) and an acoustic piano, and the jig has a shutter guide (20/ 21) assembled with a casing (15b/ 15c) of optical sensor array (15) for aligning slits (23) with target trajectories of shutter plates (11) and pieces of adhesive double coated tape (30) adhered to the shutter plates (11) for aligning the shutter plates (11) with associated white/ black keys (1), wherein the shutter plates (11) are inserted into the slits (23) so that the shutter plates 811) are placed on the target trajectories, and the white/ black keys 81) are moved along the target trajectories until they are brought into contact with the pieces of adhesive double coated tape (30) for temporality tacking the shutter plates (11) to the associated white/ black keys (1).