A detecting device has: a detecting element to which a first constant voltage is applied; a resistance element connected to the detecting element; a switching element having a first terminal to the resistance element, a second terminal controlled to a second constant voltage lower than the first constant voltage, and a control terminal sets the first terminal and the second terminal in a conducting state; a control unit, according to a conducting/non-conducting state, controls voltage to the control terminal to maintain a potential difference between the detecting element and the resistance element; and an AD converter converting, into a digital value, a potential of a potential difference between the first constant voltage and the first terminal being voltage-divided at the detecting element and the resistance element to the detecting element, a first reference potential is the first constant voltage, and a second reference potential is voltage to the first terminal.