In recent years, structural adhesives have been used to improve the rigidity, shock resistance, etc. of joints, and the requirements for these characteristics are expected to expand further. However, heat, loads, water, etc. can become deterioration factors for adhesives, and the consequent loss of strength is known to occur. In this study, the author has focused on water absorption deterioration, considered as one of the largest deterioration factors for adhesives, and has succeeded in providing high-water resistance to a one-component(1K) heat-curing epoxy adhesive through the addition of appropriate additives. This adhesive exhibited no hydrolysis during the 14-day 70 °C, 100% RH deterioration acceleration test, and strength retention and cohesive failure rates of 100% have been confirmed. In this study, the factors that provide high durability to the adhesive are identified, and the mechanism of how these factors suppress water absorption deterioration has been estimated.