The effect of a super-absorbent polymer (SAP) on the water-retention characteristics of a model sand layer was studied. A commercially available SAP (Grass Power P-200, supplied by Kurita Kogyo Ltd., Japan) was mixed with a model sand (Toyoura sand) to give an SAP content in the range of 0.05-0.4 wt%. The water-holding capacity of the mixed sand layer was found to increase with increasing SAP content, while the shape of the water characteristic curve was largely unaffected by the SAP content. This result indicates that the water retention capacity of the SAP mixed sand layer is the sum of the water retention capacity of the SAP phase and that of the sand phase. However, the water holding capacity of the SAP phase was reduced by the excess pressure exerted by the weight of the sand layer above the SAP. The effect of the excess pressure on the water-retention capacity was experimentally studied and analyzed, and modeled based on the Flory-Huggins gels model. By considering this excess pressure effect on the water retention capacity of the SAP phase, the water-holding characteristic curve of the SAP mixed sand layer can be well simulated.