PURPOSE: To shorten interpolation processing time, by calculating relative coordinates between sampling points of a hand-writing pattern inputted from a tablet, previously storing interpolation data corresponding to each relative coordinates and then reading out these stored inerpolation data by the successively calculated relative coordinates.

CONSTITUTION: A tablet input part 1 inputs a signal Si indicating the upper and lower parts of a pen and X, Y coordinate signals Xi, Yi indicating the position Pi of the pen at a fixed period, an interpolation discriminating part 2 discriminates whether the interpolation between sampling points is necessary or not from the signals Si, Si+1, and if it is necessary, an interpolation preprocessing part 6 calculates the relative coordinates of Pi+1 defining that the coordinates Xi, Yi of the sampling point Pi just before the Pi+1 is an original point. When the relative coordinates are included in a range previously stored in an interpolation data storing part 7, corresponding encoded interpolation data are retrieved, the detected output is applied to a decoding part 8 to convert the coordinates of the interpolation point into absolute coordinates by using the coordinates of the sampling point Pi from the preprocessing part 6, and the absolute coordinates are stored in a pcture storing part 9 and displayed on a display part 10. If the interpolation range exceeds a fixed range, the range is divided to be interplated.