The application of robotic manipulator has been expanded from the stand-alone operation in the strictly separated from the human workspace to the collaborative operation where it shares the workspace with human. Following this change, safety issue caused by collision between robot and human is focused on. This paper introduces risk reduction method which ensures that the contact force and pressure do not exceed the human injury threshold when robot-human collision by controlling the speed. To evaluate the collision safety of robot, contact pressure and force are calculated considering the part of robot in contact with a human. In order not to exceed the threshold pressure specified in ISO/TS 15066, a robot safety control algorithm is presented which controls the velocity of impactor. The effectiveness of the algorithm is verified with several simulation results.