Abstract: A detecting electrode disposed at a position opposed to an article to be conveyed and an auxiliary electrode shielded from an external electric field are connected to a field effect transistor through switch means. When the article to be conveyed passes the front of the detecting electrode, charge is induced at the detecting electrode, the gate electric field of the field effect transistor is varied according to the quantity of charge to produce an output responsive to the quantity of charge. When the switch means is switched to the auxiliary electrode, a zero potential is detected. Therefore, the charged potential of the article to be conveyed can be accurately measured.