In an information input/output system, a user device inputs and outputs information to and from external sources including a key management device. The system includes the key management device that securely outputs invalid-device information specifying an invalid device unit that has been made invalid for use, and the user device that includes an input/output unit and a host unit. Via the input/output unit, input and output of information is enabled between the host unit and the external sources. The host unit securely receives, via the input/output unit, the invalid-device information outputted by the key management device and judges whether or not the input/output unit is an invalid device unit by referring to the received invalid-device information. When judging, that the input/output unit is an invalid device, the host unit thereafter prohibits input and output via the input/output unit. Due to this construction, the host unit can correctly judge whether or not the input/output unit is an invalid device unit.