Underfilling material for semiconductor package

An underfilling material for a semiconductor package holding semiconductor elements on a carrier substrate mounted on a circuit board, containing a one-pack type thermosetting urethane composition which preferably comprises a urethane prepolymer having a terminal isocyanate group, which is obtained by reacting a polyol with an excessive amount of a polyisocyanate, and a fine powder-coated curing agent comprising a curing agent which is in a solid state at room temperature and surface active sites of which are covered with a fine powder. This composition can achieve both the low temperature curing properties and the storage stability.