For stimulating cooperation among users in mobile ad hoc network (MANET), an incentive mechanism which offers electric power as incentive in exchange for relaying packets by using wireless power transmission (WPT) technology has been proposed. To evaluate this mechanism, it is important to reveal a relation between the user's willingness to forward packets and the residual battery level. In this paper, we investigated and analyzed user's willingness in several situations through subjective evaluations with and without this incentive model. On the basis of the analysis, we characterize user's willingness to forward packets depending on the residual battery level in each situation. Finally, we confirmed that the incentive mechanism can stimulate user's cooperation in MANET.