A key drive device (10; 10A; 10B) which is to be installed on a keyboard musical instrument configured such that a part of a keyboard is located above a leg block via a key bed and which is configured to drive a plurality of keys of the keyboard in accordance with control signals inputted to the key drive device (10; 10A; 10B), comprising: a plurality of drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 10B-U) each of which includes a plurality of actuators for respectively driving the plurality of keys and a control circuit for controlling operations of the plurality of actuators in accordance with the control signals and each of which is to be installed on the keyboard musical instrument such that at least a part of each of the drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 10B-U) is inserted into a removal space formed by removing a part of the key bed so as to penetrate therethrough in a vertical direction, wherein at least one of the drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 1OB-U), which drives at least one of the keys, being located above the leg block while being installed on the keyboard musical instrument, includes a first portion (100b; 100Ab; 100Bb) which accommodates a part of the plurality of actuators; and a second portion (100a, 200a; 100Aa, 200Aa; 100Ba, 200Ba) which accommodates the remaining part of the plurality of actuators and the control circuit, wherein at least a part of the first portion (100b; 100Ab; 100Bb) is inserted into an insertion space portion of the removal space in a state in which the at least one of the drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 10B-U) is installed on the keyboard musical instrument, the removal space being defined by and between the leg block and the keyboard; wherein at least a part of the second portion (100a, 200a; 100Aa, 200Aa; 100Ba, 200Ba) is disposed in other space portion of the removal space except for the insertion space portion in the state in which the at least one of the drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 10B-U) is installed on the keyboard musical instrument; and wherein a dimension of the second portion (100a, 200a; 100Aa, 200Aa; 100Ba, 200Ba) as measured in at least one of a vertical direction and a horizontal short-side direction being made larger than a dimension of the first portion as measured in the at least one of the vertical direction and the horizontal short-side direction, the horizontal short-side direction being a direction perpendicular to a key arrangement direction of the plurality of keys. a plurality of drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 10B-U) each of which includes a plurality of actuators for respectively driving the plurality of keys and a control circuit for controlling operations of the plurality of actuators in accordance with the control signals and each of which is to be installed on the keyboard musical instrument such that at least a part of each of the drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 10B-U) is inserted into a removal space formed by removing a part of the key bed so as to penetrate therethrough in a vertical direction, wherein at least one of the drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 1OB-U), which drives at least one of the keys, being located above the leg block while being installed on the keyboard musical instrument, includes a first portion (100b; 100Ab; 100Bb) which accommodates a part of the plurality of actuators; and a second portion (100a, 200a; 100Aa, 200Aa; 100Ba, 200Ba) which accommodates the remaining part of the plurality of actuators and the control circuit, wherein at least a part of the first portion (100b; 100Ab; 100Bb) is inserted into an insertion space portion of the removal space in a state in which the at least one of the drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 10B-U) is installed on the keyboard musical instrument, the removal space being defined by and between the leg block and the keyboard; wherein at least a part of the second portion (100a, 200a; 100Aa, 200Aa; 100Ba, 200Ba) is disposed in other space portion of the removal space except for the insertion space portion in the state in which the at least one of the drive units (10-L, 10-U; 10A-L, 10A-U; 10B-L, 10B-U) is installed on the keyboard musical instrument; and wherein a dimension of the second portion (100a, 200a; 100Aa, 200Aa; 100Ba, 200Ba) as measured in at least one of a vertical direction and a horizontal short-side direction being made larger than a dimension of the first portion as measured in the at least one of the vertical direction and the horizontal short-side direction, the horizontal short-side direction being a direction perpendicular to a key arrangement direction of the plurality of keys.