A piezoelectric sheet included in a touch panel is provided with a split electrode having a plurality of electrode parts. Lattice-shaped coordinates are set in the touch panel, and voltages generated in the respective electrode parts through pressing manipulations with a predetermined load on the respective lattice points are preliminarily stored in a storage portion as reference voltages. During actual use, actually-measured voltages induced in the respective electrode parts due to a pressing manipulation are detected by a detection portion. The ratios of the measured voltages to the reference voltages are calculated by an operation portion, and the average thereof and the standard deviation there among are determined. The coordinates of the pressed position are determined from the coordinates of four lattice points in the ascending order of standard deviations. The load is multiplied by the ratio to determine the pressing force of the lattice point having a minimum standard deviation.