An electronic keyboard musical instrument, comprising: an instrument main body (1, 2, 3, 4; 81, 82, 83, 88, 90, 91, 93; 111, 112, 113, 114, 118, 124; 201, 202, 203, 204, 208, 214; 321, 322, 326, 327, 329, 332; 401, 402, 406, 407, 409, 421, 422; 610); a keyboard portion (4; 114; 204; KB) including a plurality of keys as performance operating elements; a musical-sound-signal generating portion (63; 123; 213; 303; 493) configured to generate a musical sound signal by a key operation of the keyboard portion; and at least one planar speaker (7; 30; 40; 50A, 50B; 95; 132; 222; 290; 338; 350; 372; 380; 414; 512; 630) configured to output a sound in accordance with the musical sound signal generated by the musical-sound-signal generating portion, and a left leg body and a right leg body which support the instrument main body at a left portion and a right portion of the instrument main body, respectively; wherein the planar speaker is formed by superposing a plurality of flexible layers that at least include a vibrating layer and electrode layers which sandwich the vibrating layer from opposite sides of the vibrating layer, and wherein the planar speaker is disposed at an inside of a casing that constitutes the instrument main body or disposed at a portion of a surface of the casing, characterized in that the electronic keyboard musical instrument further comprises guide members provided on the left leg body and the right leg body, respectively, wherein the planar speaker is constituted by a plurality of planar speaker panels (413 1, 413 2, 413 3, 413 4) each of which is constituted such that respective peripheries of a plurality of planar speaker portions (414 1, 414 2, 414 3, 414 4) of the planar speaker are supported by respective frames (415 1, 415 2, 415 3, 415 4), wherein a connected planar speaker panel which is constituted by at least two of the plurality of planar speaker panels is constituted such that respective frames of any adjacent planar speaker panels are connected by connecting members (424, 425, 426), wherein an upper end of the connected planar speaker panel is held between the left leg body and the right leg body so as to be a fixed end while a lower end of the connected planar speaker panel is a free end, wherein the connected planar speaker panel includes, at a left end and a right end thereof, guided members, the movements of which are regulated by the guide members provided on the left leg body and the right leg body, respectively, and wherein the connected planar speaker is configured such that the guided members move relative to the corresponding guide members while the movements of the guided members are regulated by the corresponding guide members, so that a state of the planar speaker changes. an instrument main body (1, 2, 3, 4; 81, 82, 83, 88, 90, 91, 93; 111, 112, 113, 114, 118, 124; 201, 202, 203, 204, 208, 214; 321, 322, 326, 327, 329, 332; 401, 402, 406, 407, 409, 421, 422; 610); a keyboard portion (4; 114; 204; KB) including a plurality of keys as performance operating elements; a musical-sound-signal generating portion (63; 123; 213; 303; 493) configured to generate a musical sound signal by a key operation of the keyboard portion; and at least one planar speaker (7; 30; 40; 50A, 50B; 95; 132; 222; 290; 338; 350; 372; 380; 414; 512; 630) configured to output a sound in accordance with the musical sound signal generated by the musical-sound-signal generating portion, and a left leg body and a right leg body which support the instrument main body at a left portion and a right portion of the instrument main body, respectively; wherein the planar speaker is formed by superposing a plurality of flexible layers that at least include a vibrating layer and electrode layers which sandwich the vibrating layer from opposite sides of the vibrating layer, and wherein the planar speaker is disposed at an inside of a casing that constitutes the instrument main body or disposed at a portion of a surface of the casing, characterized in that the electronic keyboard musical instrument further comprises guide members provided on the left leg body and the right leg body, respectively, wherein the planar speaker is constituted by a plurality of planar speaker panels (413 1, 413 2, 413 3, 413 4) each of which is constituted such that respective peripheries of a plurality of planar speaker portions (414 1, 414 2, 414 3, 414 4) of the planar speaker are supported by respective frames (415 1, 415 2, 415 3, 415 4), wherein a connected planar speaker panel which is constituted by at least two of the plurality of planar speaker panels is constituted such that respective frames of any adjacent planar speaker panels are connected by connecting members (424, 425, 426), wherein an upper end of the connected planar speaker panel is held between the left leg body and the right leg body so as to be a fixed end while a lower end of the connected planar speaker panel is a free end, wherein the connected planar speaker panel includes, at a left end and a right end thereof, guided members, the movements of which are regulated by the guide members provided on the left leg body and the right leg body, respectively, and wherein the connected planar speaker is configured such that the guided members move relative to the corresponding guide members while the movements of the guided members are regulated by the corresponding guide members, so that a state of the planar speaker changes.