PURPOSE: To accurately remove the erroneously measured data of the coordinate detection device.

CONSTITUTION: An intermediate point Ci between the coordinates Pi-1 detected just before the coordinate Pi which is read from a tablet controller and the coordinates Pi+1 which is detected just after the coordinates Pi is obtained (step S2). The distance di between the intermediate point Ci and the coordinates Pi is obtained (step S3) and the distance li between the coordinates Pi-1 and the coordinates Pi+1 is obtained (step S4). Whether or not the coordinates Pi is effective is discriminated based on the distance di and the distance li (step S5).