PURPOSE: To make capacity of a storage device extremely small so as to make a device an inexpensive one by dividing a range 0W90°, which a phase can have, into n-number of sections, reading out the parameter corresponding to the section and obtaining the position of a coordinate.

CONSTITUTION: At first, to what section the phase θ detected by an input pen 5 belongs is decided. namely, (i) which satisfies the section Pi-1≤θ<Pi is obtained. Next, the parameters Ai and Bi corresponding to the said section is fetched from a storage device 6 according to (i). By executing a computation such as Ai×θ+Bi to the fetched Ai and Bi, the position X corresponding to the phase θ can be obtained. Since the position X is obtained only by providing a transformation table written in the parameters Ai and Bi corresponding to the section (i) and a section end point Pi, whose number is as same as the number (n) of the section (i), even if the resolution of the phase θ and the position X rise, in the storage device 6, the capacity of storage is made small and the device is made inexpensive.