To provide a one liquid type pr two liquid type sealing material composition useful as a substitute for a polysulfide-based sealing material that has been widely for jointing work from the application in a factory to the application at a construction site.

This one liquid type sealing material composition is characterized by using a caster oil-based silage-modified urethane prepolymer obtained by reacting a caster oil-based terminal isocyanate group-containing urethane prepolymer obtained by the reaction of a polyol containing the castor oil-based polyol at least in part with excess polyisocyanate compound, with a mercaptolkoxysilane; and the two-liquid type sealing composition is characterized in consisting of a main agent component of a silane-modified urethane prepolymer obtained by reacting a terminal isocyanate group-containing urethane prepolymer with a mercaptoalkoxysilane, and a curing agent component consisting of a polyol containing a castor oil-based polyol at least in a part and a curing-accelerating catalyst.