In response to a command to start restrictions on a communication service of a computer, the communication service is restricted by a countermeasures apparatus which replaces the communication address of a second computer, which has been stored in a first computer, with the communication address of the countermeasures apparatus, and replaces a communication address of the first computer, which has been stored in the second computer, with the communication address of the countermeasures apparatus. Accordingly, the countermeasures apparatus acquires a packet from the first computer to the second computer and determines whether or not this acquired packet is to be transmitted to the second computer.