An efficient method to compute the joint friction torques as well as the external contact forces in forward dynamics simulation of a robot manipulator is proposed. The existence of friction inside mechanical gears substantially affects behaviors of the robot, and hence, should be taken into account. The strict form of the equation of motion disables distinguishing the internal friction torques from the external forces, which causes an increase of the degree of indeterminacy and the computation cost. A semi-implicit approach in which the internal friction torques and the external contact forces are alternatively computed is proposed based on an assumption that the net joint actuation torque does not much vary in a short interval. It significantly reduces the computation cost at a comparative accuracy with a completely implicit method.