A portable file server includes a radio field intensity input unit, a current controller, and an antenna. A radio field intensity for use in wireless communication between the portable file server and a terminal through the antenna is inputted to the radio field intensity input unit. The current controller increases or decreases the amount of signal current for data to be transmitted to the terminal in accordance with the radio field intensity inputted to the radio field intensity input unit. The signal current is, in turn, outputted to the antenna. Thus, the radio field intensity is controlled such that the radio waves can be received only in the area where wireless communication between the portable file server and the terminal takes place; thus, unauthorized wireless access from outside of this area will be prevented. Hence, the portable file server provides enhanced security.