54 thereafter the control goes back to the step s 50 return when the counter y is equal to or smaller than the variable a no at step s 52 the control also goes back to the step s 50 return on the other hand when no variation detection notice is received no at step 650 the statistical processing section 3 1 determines whether the counter y is 0 stop s 55 if the counter y is not 0 no at step s 55 it is determined that the phenomenon of variation that has occurred stops and the statistical processing section 31 1 calculates a variable x indicative of a proper variation buffer value at that time by the following expression where y is a counter indicative of the number of times a notice of occurrence of variation has been received z is a current variable buffer value and b is a minimum correction value of variation buffer value stop s 56 then the statistical processing section 31 1 instructs the buffer value correcting section 32 1 to replace the variation buffer value with the calculated proper variation buffer value x step s 57 then the counter y is reset to 0 step s 54 and the control goes back to the step s 50 return when the counter y is 0 yes at step s 55 the current variation buffer value is set to the basic value step s 58 and the control goes back to the step s 50 return as described above the variation buffer value gradually increases from the basic variation buffer value initial value depending on a status of occurrence of cell.