An acoustic upright piano, an electronic sound system and a hammer stopper form in combination a keyboard musical instrument for generating acoustic sounds in an acoustic sound mode and electronic sounds in an electronic sound mode, and the hammer stopper has a rotatable shaft extending in a lower space under damper blocks of damper assemblies, cushion members projecting into an upper space over the damper blocks and connecting brackets provided between the rotatable shaft and the cushion members, thereby perfectly interrupting hammer motions before an impact on strings in the electronic sound mode.