PURPOSE: To attain accurate input operation by allowing a position converting part to convert a position touched on a touch screen into a position on a display screen based upon its relative relation.

CONSTITUTION: When a reference positioning mode is selected by a reference positioning instructing part 12, touch images are displayed on reference positions A, B of the display screen 1 of a display device 11. When the touch images are touched, the position (coordinate) data are detected by the touch screen 2 and averaged by a conversion factor computing part 14 and relative relation between a position on the display screen and a position on the touch screen 2 is determined, and the data are stored in the position converting part 15. Thereafter, the coordinate data are transferred to the converting part 15 by touching a position on the touch screen 2 and converted into the position on the display screen by using the relative relation determined by the computing part 14. Consequently, low operability can be improved and accurate touch screen input operation can be attained.