To investigate the mechanism of seismic damage to irrigation tanks, the irrigation tanks damaged during the Mid Niigata Prefecture earthquake in 2004 were analyzed. The damaged irrigation tanks were classified into damage conditions. It was found that a large embankment located near the seismic center had a high risk for seismic disaster. The circular slice method was applied to damaged and undamaged irrigation tanks. The slide at the upstream slope of the embankment was considered as a possible failure pattern. Sensitivity analyses of the parameters used in the calculation of safety factors were also conducted to examine damage prevention. Since the slope angle was as influential as the internal friction angle, widening of the embankment would be effective for earthquake resistance. The conditional damage probability of a given irrigation tank was calculated using a Monte Carlo simulation, which is useful for risk management.