To improve the resolution of the coordinate data to be inputted and also to decide the hand shake input by storing the first to third coordinate data which are detected in time series and deciding the hand shake input when each of different data of the coordinate data are less than the prescribed value.

Each of light receiving parts 60X and 60Y has many photodetectors 63 in the slits 62 to receive the reflected light of a coordinate input pen P and detects the addresses of intermediate value of plural photodetectors 63 where the light receiving signals are obtained to detect the current coordinates of the pen P. Then the first to third coordinate data which are detected in time series are stored and the first difference data, i.e., the difference between the first and second coordinate data are calculated together with the second difference data between the first and third coordinate data and the third difference data between the second and third coordinate data respectively. The hand shake input is decided when the both first and second difference data or both first and third difference data are less than the prescribed value.