Kinetic theoretical approach is employed in analyzing the flow behavior of a rapidly deforming granular disk assembly. Binary collisions are assumed and two types of energy dissipation mechanisms are carefully investigated. The roughness coefficient that relates the relative velocities at contact before and after a collision is assumed to depend upon the orientation of the colliding disks. The present theory takes account of the geometrical dependence of a collision in the averaging scheme and estimates the constitutive relations accordingly.