It is commonly said that the major objective of disaster prevention is to reduce social damages caused by disasters. From this viewpoint this paper examines methods to evaluate social damages caused by disasters. Social damages caused by disasters are often evaluated by the expected losses to society (social losses). The social loss is defined as the decrease in social welfare measured in monetary term from ordinary to disaster state. It is claimed that "ex ante" and "ex post" evaluation of disaster-caused damages should be strictly distinguished. Ex ante evaluation which is conducted in planning disaster prevention facilities requires the probability distribution of social damages to be identified a priori. The paper demonstrates that the social expected loss as an evaluation criterion may well reflect the characteristics of the disasters but fail to evaluate the social preferences of the probability distributions of the resultant social damages.