To obtain a one pack type urethane adhesive composition excellent in initial adhesion force by including an active isocyanate group-containing urethane prepolymer and an epoxylated natural rubber at a specific ratio.

This adhesive composition comprises (A) 100 pts.wt. active isocyanate group-containing urethane prepolymer, (B) 0.2-5.0 pts.wt., preferably 1.0-3.0 pts.wt. epoxylated natural rubber and (C) as necessary, 10-30 pts.wt., preferably 15-20 pts.wt. rosin polyol (e.g. a rosin diol). The component A is a reaction product obtained by heating (i) a polyol compound, (ii) an excess polyisocyanate compound in an amount of 2-15 equivalent (NCO equivalent) per equivalent of the component (i) under stirring and contains preferably 3-15 wt.% isocyanate group at the molecular ends. The component B includes a material obtained by treating a natural rubber with peracetic acid and binding epoxy group thereto.