A modular keyboard includes an upper casing, a lower casing corresponding to the upper casing, an electric conduction element and a circuit board. The upper casing includes a first side and a second side opposite to each other, and a plurality of press keys disposed on the first side. The lower casing is set for sealing the second side, and an enclosure space is formed between the upper casing and the lower casing. The electric conduction element is installed on the second side in the enclosure space, and the circuit board is installed between the electric conduction element and the lower casing. With a supersonic welding technique, the upper casing and the lower casing are engaged with each other to seal the enclosure space and isolate the electric conduction element in the enclosure space and the circuit board from the exterior.