To provide an information processing device and an antenna extension system which are capable of transmitting/receiving a radio wave to/from a portable module even when the portable module is built in the information processing device.

An information processing device 1 includes: a card connector 4 to which a card module 2 having an antenna 103 built therein is mounted; and an antenna 5 for transmitting/receiving a high-frequency signal to/from the antenna 103. When the card module 2 is mounted to the card connector 4, the antenna 5 is fixed to the position on a substrate 8 which faces the antenna 103, and is connected to an antenna 6 having a gain larger than that of the antenna 5.