In Japan, earthquake resistance improvement (earthquake proofing) of drinking water infrastructure is not as developed as for other types of infrastructure such as gas and electricity. To facilitate earthquake proofing, it is important for water utilities to encourage customers to better understand its importance and effectiveness. So that water consumers could instinctively understand the effectiveness of earthquake proofing, we applied the concept of ‘recovery time expectancy’ as an indicator of earthquake resistance of water treatment plants. In this study, we performed a recovery simulation of a medium-scale water treatment plant based on the system reliability theory, and then evaluated the effectiveness of earthquake proofing using recovery time expectancy as an indicator. The simulation was performed with five cases: the present condition of the facility and four hypothetical cases with different levels of earthquake proofing. As a result, the effectiveness of earthquake proofing was able to be expressed by a recovery curve showing the reduction of recovery time expectancy, and the priority became clear for seismically proofing each component of the plant according to the earthquake scale. Finally, the optimum capital investment was determined by comparing total costs (damage cost plus investment cost) of the five cases, which were computed according to recovery time expectancy after earthquake proofing.