In a tentative target value calculation section 28, a predetermined value is subtracted from (or added to) a target value Exp to calculate a tentative target value ExpB. In a binary search executing section 25, binary search is executed, and a searching region is limited to a certain region including this tentative target value ExpB. Next, in a sequential search executing section 29, the target value Exp is searched for in an increasing direction from the tentative target value ExpB which is a start point in the limited searching region. Accordingly, both drop prevention of measurement precision and reduction of searching time are achieved consistently, and a target value is securely and normally found in a case where a sequence constituting a searching object indicates an ascending-order sequence including a decrease in a part.