A physical quantity detecting apparatus includes a plurality of physical quantity conversion circuits, an output selection circuit and a signal conversion circuit. Each of the plurality of physical quantity conversion circuits converts a physical quantity to be detected into a voltage corresponding to the physical quantity and outputs the voltage. The output selection circuit is electrically connected to the plurality of physical quantity conversion circuits to select a maximum voltage from among the voltages outputted from the plurality of physical quantity conversion circuits. The signal conversion circuit is electrically connected to the output selection circuit to convert the voltage outputted from the output selection circuit into a pulse signal having a pulse width or frequency corresponding to the voltage and output the pulse signal.