A keyboard apparatus comprising: a frame having hammer supports and integrally formed by resin, said frame being adapted to be supported on a musical instrument main body; a plurality of white and black keys (10, 20) mutually juxtaposed on said frame, each of said black keys having a part to be arranged in a visible way forward of a panel of said musical instrument main body such that the visible parts of said black keys are visible during performance; a plurality of hammers mutually juxtaposed so as to correspond to respective ones of said keys (10, 20) and supported by the hammer supports of said frame at locations below respective ones of said keys (10, 20) so as to each pivot about a corresponding one of the hammer supports in conjunction with the corresponding key (10, 20) and impart inertia to a pivotal motion of the key (10, 20); detection devices (58, 59) each adapted to be depressed by a corresponding one of said keys (10, 20) and detect an operation of the key (10, 20); and detection-device mounting portions (56, 57) integrally formed on said frame and respectively mounted with said detection devices (58, 59), wherein said hammers have their rear ends (32p1) located forward of rearmost positions (26p1) of the visible parts of said black keys, the hammer supports are located rearward of frontmost positions (26p2) of the visible parts of said black keys, characterized in that said black and white keys (10, 20) form a key unit (WU1, WU2, BU) by having key main bodies (16, 26) thereof each connected to its respective base end (18A, 18B, 28) via a thin plate-like hinge (17, 27), each of the key main bodies (16, 26) being pivotable about each respective base end (18A, 18B, 28) as a key fulcrum in key depression and release directions, said keys (10, 20) are mounted to said frame by being fastened to a fastening part of said frame by a fastener, with the base ends (18A, 18B, 28) of said keys stacked one upon another, the hinges (17, 27) of said keys (10, 20) are formed to extend in a direction to cross key depression surfaces of the key main bodies (16, 26) and parallel to a key arrangement direction, each of said hammers is adapted to be driven at its front half located forward of the hammer support such that a rear end portion of the hammer is moved upward in a key-depression forward stroke and said detection-device mounting portions (56, 57) are located forward of the rearmost positions (26p1) of the visible parts of said black keys and upward of the fastening part of said frame. a frame having hammer supports and integrally formed by resin, said frame being adapted to be supported on a musical instrument main body; a plurality of white and black keys (10, 20) mutually juxtaposed on said frame, each of said black keys having a part to be arranged in a visible way forward of a panel of said musical instrument main body such that the visible parts of said black keys are visible during performance; a plurality of hammers mutually juxtaposed so as to correspond to respective ones of said keys (10, 20) and supported by the hammer supports of said frame at locations below respective ones of said keys (10, 20) so as to each pivot about a corresponding one of the hammer supports in conjunction with the corresponding key (10, 20) and impart inertia to a pivotal motion of the key (10, 20); detection devices (58, 59) each adapted to be depressed by a corresponding one of said keys (10, 20) and detect an operation of the key (10, 20); and detection-device mounting portions (56, 57) integrally formed on said frame and respectively mounted with said detection devices (58, 59), wherein said hammers have their rear ends (32p1) located forward of rearmost positions (26p1) of the visible parts of said black keys, the hammer supports are located rearward of frontmost positions (26p2) of the visible parts of said black keys, characterized in that said black and white keys (10, 20) form a key unit (WU1, WU2, BU) by having key main bodies (16, 26) thereof each connected to its respective base end (18A, 18B, 28) via a thin plate-like hinge (17, 27), each of the key main bodies (16, 26) being pivotable about each respective base end (18A, 18B, 28) as a key fulcrum in key depression and release directions, said keys (10, 20) are mounted to said frame by being fastened to a fastening part of said frame by a fastener, with the base ends (18A, 18B, 28) of said keys stacked one upon another, the hinges (17, 27) of said keys (10, 20) are formed to extend in a direction to cross key depression surfaces of the key main bodies (16, 26) and parallel to a key arrangement direction, each of said hammers is adapted to be driven at its front half located forward of the hammer support such that a rear end portion of the hammer is moved upward in a key-depression forward stroke and said detection-device mounting portions (56, 57) are located forward of the rearmost positions (26p1) of the visible parts of said black keys and upward of the fastening part of said frame.