Furan resin used as a binder for casting sand molds is derived from natural plant sources, and its use contributes to carbon neutrality. However, the strength deterioration of sand molds molded with furan resin often occurs due to unknown causes. For a detailed understanding of the improvement of the strength of furan resin sand molds, the adhesion of contact points of sands was evaluated by fracture surface analysis. New silica sand and reconditioned silica sand were used as foundry sand for the furan resin mold specimens. The compressive strength of specimens was measured, and SEM evaluated the fracture surfaces near crack initiation. By increasing the amount of furan resin from 0.8–1.2%, the new sand samples showed an increase in strength by approximately 3 times and an increase in the number of contact points by about 0.5. In contrast, the recycled sand samples exhibited an increase in strength by about 1.9 times and a decrease in the number of contact points by about 1.0. This suggests that the adhesion form of furan resin influences the mold strength.