A pen-shaped input apparatus for inputting drawings, symbols, characters into a data processing device, such as a computer, is developed. It can trace a handwritten 2-dimensional trajectory using built-in inertial sensors. As sensors, it has 3 accelerometers and 3 gyroscopes. By detecting accelerations about 3 axes and angular rates around them as 6 degrees of freedom, it can represent the 2-dimensional trajectory of the pen tip after additional calculation. Velocity correction enables precise trajectories to be represented with a small integration error. Experimental results demonstrate the feasibility of this device as a handwritten input apparatus.