Geocast communication provides efficient group communication services to distribute information to terminals that exist in some geographical domain. For various services which use geocast communication, ad hoc network is useful as network structure. Ad hoc networks are a kind of self-organing network where terminals communicate directly with each other without network infrastructure. For ad hoc networks, terminal power saving is an important issue, because terminals are driven by the battery powered system. One approach for this issue is reducing the radio transmission range of each terminal, but it degrades reachability of user data for each terminal. In this paper, we propose a design method for radio transmission range using the target problem to improve both terminal power saving and reachability for geocast communication in an ad hoc network. Moreover, we evaluate the proposed method considering both routing protocols and media access control protocols, and clarify the applicability of the proposed method to communication protocols.