Even though the double bounded contingent valuation method (DBCVM) is an efficient estimation method of the willingness to pay for a non-market good, it has long been criticized to be an inappropriate estimation method because of interdependence of subsequent two answers during a course of survey. Many experimental studies have reported that two subsequent answers are not made independently, but the second answer depends upon the first answer. There are many hypotheses contending a different pattern of dependence of the answers. In this study, the dependence of two answers are tested. But no dependence pattern has been detected.