Techniques are provided for determining a position of a node. The headrest includes a headrest body that is movable with respect to the support structure between a stowed position and a use position, and a locking mechanism for locking the headrest body in the use position. The electronic unit detects an occupant based on an impedance and a phase difference, which are based on the load current and the potential current. Thus, a less-error, high-accuracy optimum value of the defocus offset amount can be obtained. When the calculated change width w is equal to or more than a reference value w0 it is judged that a quick tire deformation is currently generated and a driver of the vehicle is informed of the quick deformation by actuating a warning device.