This study examines a plan for efficient leakage investigation to decide the number of survey-subjected water meters in the Option A method, which requires considerable manpower and expense, along with possible civil complaints, among the possible methods for determining the minimum night flow. The Bayesian approach was adopted, and the prior distribution of night use and water supply pipe night flow losses were proposed in order to estimate the most efficient survey rate in Option A test. From the results, the prior distribution was decided as the gamma distribution by surveying the minimum night flow for all the water meters for district metered area A. The estimated value of the posterior distribution, i.e., the minimum night flow, was determined by using the Bayesian inference by changing the survey rate of water meters. From the results, the survey rate for maximizing the survey efficiency was calculated. The most effective survey rate was 58% for household water meters and 78% for non-household water meters.