An input apparatus for using an expansion key in a portable terminal includes an expansion key matrix unit which forms a key matrix through which a plurality of expansion keys are input; an input/output (I/O) expansion chip which, in response to a key input through the expansion key matrix unit, transmits a value of an input port of the input/output (I/O) expansion chip to store in an internal parameter, changes the input port to an output port, and vice versa, reads a value of the changed input port, and transmits the read value to the MSM chip to be stored in the internal parameter; and the MSM chip which recognizes a corresponding expansion key based on the stored parameters.