PURPOSE: To physically limit the grant/inhibition of mutual accesses for each computer main body by transmitting a ciphered access request command and decoding this command via the cipher keys of a computer of the receiver side.

CONSTITUTION: A computer C3 has the same cipher keys as a computer A1 so that the mutual accesses are granted between both computers. Meanwhile a computer B2 has the different cipher keys from the computer A1 so that the mutual accesses are inhibited between both computers. A user carries out a command for an access applied to the computer C3 from the computer A1. Then the computer A1 reads the cipher keys written into a ROM 11 to send them to a ciphering device 4 and ciphering the command to sent it to the computer C3 via a local area network 5. The computer C3 reads the cipher keys written in the ROM 11 to send them to the device 4 and decodes the ciphered command.