Robots are expected to expand their usages into more complicated tasks and in unstructured environments. As an example of such a task, peg-in-hole task by robot is studied in this paper. We report on a computer simulation and an experiment of peg-in-hole task with no edge chamfers, executed by a robot with a force sensor mounted in the wrist applying an appropriate insertion procedure and a hybrid (position/force) control. The robot is a planar Cartesian robot with three degrees of freedom (translation with two degrees of freedom, rotation with one degree of freedom) . A condition for the peg to be jammed in the hole under the force control mode is also studied. When the robot insert the peg into the hole, stronger vibrational behaviors both in the contact force and position signals appeared. Therefore we studied vibrational characteristics to find a method to reduce the vibrations.