An optical semiconductor package sealing resin material used to seal an optical semiconductor chip in a semiconductor package includes a thermosetting epoxy composition and a hydrophobic smectite clay mineral. The hydrophobic smectite clay mineral is hydrophobized by subjecting a hydrophilic smectite clay mineral to an intercalation reaction with an alkylammonium halide. The smectite clay mineral is bentonite, saponite, hectorite, vermiculite, stevensite, tainiolite, montmorillonite, or nontronite.