This paper reports on a questionnaire survey of 16 national libraries designed to clarify how national libraries attempt to justify their web archiving activities. Results indicate they envisage that a) the benefits brought about by their initiatives are greater than the overall costs, b) the costs imposed on libraries are greater than the costs imposed on stakeholders, and c) all of them are making efforts to respond to legal risks in various ways (e.g. legislation, contracting and opt-out policies) although there are trade-off relations in terms of costs for negotiation, scope of access and size and scope of the web archive. The paper discusses whether a basic logic for justification of their web archiving is valid from the perspective of balancing cost-benefit. Further, it highlights the potential, underlying premises of the logic that motivates the intervention of national libraries as public sector organizations. (Contains 3 tables and 2 figures.)