This study aimed to verify the accuracy of inundation analysis by comparison with field-survey results for a small agricultural earthfill dam that failed due to heavy rain. In Japan, dam failures are caused by concentrated heavy rain or large earthquakes. Potential inundation areas are displayed on municipal “Hazard Maps” to reduce the risk of damage to downstream sites. However, the accuracy of inundation analysis for small earthfill dams has never been verified through comparison with field surveys.The following work was completed; i) the effect on the downstream gradient was verified by a simplified (one-dimensional unsteady flow) analysis for a case of dam failure; ii) the water depth and width of flow path in inundation areas were compared based on differences in peak outflow; and iii) the effect of the culvert structure blocked by flood debris downstream was also verified by comparing the result of analysis and the field survey. The following results were obtained: i) From the simplified analysis, the downstream water depth was sensitive to changes in outflow distribution from the dam breach section: ii) In the inundation analysis, using the peak outflow prediction equation proposed by Costa (1988), the water depth was deeper at the center of the flow path and the flow path was wider: iii) The water depth around the blocked culvert structure matched the field-survey results (the elevation data of the meshes around the culvert structure were raised to simulate the blocked structure).