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 DELTAC 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 DELTAC. Consequently, change of the capacitance C0 comparable to that brought about upon detection of the
human body takes place, whereby a substantially similar detection
signal as the
human body detection
signal is made available. On the basis of this detection
signal, it is possible to monitor whether or not the sensor apparatus is operating normally.