A keyboard apparatus comprising: a key unit in which a plurality of keys (20, 40) having driving parts (29, 45) are connected by a common base end part in a condition in which said keys (20, 40) are pivotally movable; a support member (10, 85) having a key base end part securing part (15, 16); a plurality of mass bodies having driven parts (53,54) disposed in correspondence to said driving parts (29, 45) of said keys (20, 40), respectively, and which are disposed in a pivotally movable condition with respect to said support member (10, 85); a key mounting guide part (19, 89a) that guides said driving parts (29, 45) of said keys (20, 40) of said key unit to said corresponding driven parts (53, 54) of said mass bodies in a process for attaching said key unit; and a mass body mounting guide part (19,89) for guiding said plurality of mass bodies into a pivotally movable condition in said support member and also for serving as said key mounting guide part (19,89) wherein an assembled state is achieved when said common base end part of said key unit is secured to said key base end part securing part (15, 16) of said support member (10, 85) and said driving parts (29, 45) of said keys (20, 40) of said key unit are engaged with said corresponding driven parts (53, 54) of said mass bodies, respectively; and the keyboard apparatus being configured such that, in the assembled state, in response to a pivotal movement of each of said keys (20, 40), said corresponding driven parts (53,54) of said mass bodies are driven by said driving parts (29, 45) of said keys (20, 40), respectively, so that said corresponding mass bodies pivotally move; wherein said keys (20, 40) are integrally connected by said common base end part, and wherein said keyboard apparatus is configured such that, by moving said key unit in a longitudinal direction of said keys (20, 40) while adjusting each position of the keys in said key unit in a key-alignment direction of said key unit in a state in which said plurality of mass bodies are disposed in said support member (10, 85), said driving parts (29, 45) of all of said keys (20, 40) of said key unit simultaneously engage with said corresponding driven parts (53, 54) of said mass bodies, respectively, and, when said driving parts (29, 45) are simultaneously engaged with said driven parts (53, 54), said common base end part of said key unit is positioned at a position in which said common base end part can be secured to said key base end part securing part (15,16) of said support member (10, 85), wherein the longitudinal direction of said keys (20, 40) is a rearward direction of the keyboard apparatus, the keyboard apparatus characterised in that said key mounting guide part (19, 89) slopes downward towards a side of each of said driven parts (53, 54) of said mass bodies. a key unit in which a plurality of keys (20, 40) having driving parts (29, 45) are connected by a common base end part in a condition in which said keys (20, 40) are pivotally movable; a support member (10, 85) having a key base end part securing part (15, 16); a plurality of mass bodies having driven parts (53,54) disposed in correspondence to said driving parts (29, 45) of said keys (20, 40), respectively, and which are disposed in a pivotally movable condition with respect to said support member (10, 85); a key mounting guide part (19, 89a) that guides said driving parts (29, 45) of said keys (20, 40) of said key unit to said corresponding driven parts (53, 54) of said mass bodies in a process for attaching said key unit; and a mass body mounting guide part (19,89) for guiding said plurality of mass bodies into a pivotally movable condition in said support member and also for serving as said key mounting guide part (19,89) wherein an assembled state is achieved when said common base end part of said key unit is secured to said key base end part securing part (15, 16) of said support member (10, 85) and said driving parts (29, 45) of said keys (20, 40) of said key unit are engaged with said corresponding driven parts (53, 54) of said mass bodies, respectively; and the keyboard apparatus being configured such that, in the assembled state, in response to a pivotal movement of each of said keys (20, 40), said corresponding driven parts (53,54) of said mass bodies are driven by said driving parts (29, 45) of said keys (20, 40), respectively, so that said corresponding mass bodies pivotally move; wherein said keys (20, 40) are integrally connected by said common base end part, and wherein said keyboard apparatus is configured such that, by moving said key unit in a longitudinal direction of said keys (20, 40) while adjusting each position of the keys in said key unit in a key-alignment direction of said key unit in a state in which said plurality of mass bodies are disposed in said support member (10, 85), said driving parts (29, 45) of all of said keys (20, 40) of said key unit simultaneously engage with said corresponding driven parts (53, 54) of said mass bodies, respectively, and, when said driving parts (29, 45) are simultaneously engaged with said driven parts (53, 54), said common base end part of said key unit is positioned at a position in which said common base end part can be secured to said key base end part securing part (15,16) of said support member (10, 85), wherein the longitudinal direction of said keys (20, 40) is a rearward direction of the keyboard apparatus, the keyboard apparatus characterised in that said key mounting guide part (19, 89) slopes downward towards a side of each of said driven parts (53, 54) of said mass bodies.