Abstract: A sensor apparatus and a safety apparatus which can ensure significantly enhanced safety by monitoring constantly whether the sensor apparatus is operating normally. An auxiliary electrode plate of a small area is disposed in parallel and in opposition to an electrode plate with a short distance therefrom. A switching element is turned on every second for a period of milliseconds repetitively. When the switching element is turned on under the control of a central processing unit, the auxiliary electrode plate is connected to the ground potential, as a result of which a capacitance &Dgr;C effective between the auxiliary electrode plate and the electrode plate is electrically connected in parallel with a capacitor C0 effective between the electrode plate and a machine component, resulting in that the capacitance C0 is increased by a proportion corresponding to the capacitance &Dgr;C.