An instruction inputting device detects input operations in the XY plane and in a rotational direction on a hemispherical input operation surface. Four detection electrodes insulated from each other are formed at 90° intervals around the Z axis of the hemisphere, on the surface of a spherical crown-shaped insulator. Surface areas of each of the detection electrodes decrease moving toward an apex P on the Z axis and moving in a circumferential direction away from the positions at which the 90° intervals are formed. As a result, the surface areas of the detection electrodes facing an object to be detected for causing the input operation differ depending on the positioning of the object, thereby causing differences in the floating capacitances of the detection electrodes which are sensed by a circuit portion of the device.