An unauthorized access prevention system includes a countermeasure method determination unit determining a method of taking countermeasures to protect a destination against unauthorized access based on a rate indicating a probability that predetermined traffic is unauthorized access. The rate is determined by calculating a number of times that the countermeasures were taken and subtracting a number of times that countermeasures were suspended, and dividing the resultant number of times that countermeasures were taken without suspension by the number of times that countermeasures were taken, for the predetermined traffic. The system also includes a storage device storing the number of times that countermeasures were taken and the number of times that countermeasures were taken without suspension. A countermeasure execution control unit controls when to take the countermeasures in the determined method. The countermeasure method determination unit selects an appropriate method of taking countermeasures by comparing the rate to predetermined threshold values.