PURPOSE: To prevent a DC voltage from being continuously impressed to liquid for reflection prevention sealed in a tablet.

CONSTITUTION: A counter 2 and an OR gate 73 generate a control signal REV 0 of which the level is inverted in a prescribed cycle during an input waiting period. A changeover switch 66 inverts a voltage level to be impressed to terminals XH and YH of a tablet 51 synchronously with the control signal REV 0. Switches 67 and 68 and an inverter 69 invert a terminal for extracting a control signal MON to be transmitted to a coordinate detecting switch 60 to the side of a terminal YL and a terminal XL synchronously with the control signal REV 0. Thus, by inverting the level of the voltage to be impressed to the tablet 51, the voltage to be loaded to the liquid for reflection prevention sealed in the tablet 51 is made into AC. Therefore, upper and lower conductive films or the liquid for reflection prevention can be prevented from being degraded.