When the computer is powered on, a POST program is executed, and in the setting for enabling a security function, an RFID chip 33 makes Removal Detect Enable a high level and outputs it to the control side of an analog switch 67 and one input of a NAND element 63. The removal of an RF antenna 37 causes a first short-circuit element 36 to be disconnected for shutting off a terminal 71 and a terminal 73, and an INTR signal for prohibiting the keyboard input is outputted by the high-level signal of the element 63 to prohibit access to the computer. If the RF antenna is removed while the power is off, the analog switch 67 is disconnected and the power supplied from a lithium battery to a CMOS memory 50 is shut off, whereby access to the computer can be prohibited.