Abstract: An input/output device of a smart card according to the present invention comprises a first register having a serial input terminal connected to a terminal serial input/output; a second register having parallel detection terminals, parallel input/output terminals connected to a data bus, and a serial input terminal connected to a serial output terminal of the first register; a third register having a parallel input terminal connected to a ground, a serial input terminal connected to a serial output terminal of the second register, and a serial output terminal connected to the terminal serial input/output; a parity generator which receives parallel output data from the parallel detection terminals of the second register, logically operates upon the parallel output data to generate an operation parity bit which is applied to a parallel input indicator terminal of the first register; and a parity detector which receives a reception parity bit from the first register and the operation parity bit from the parity g