An error detection apparatus for use with an image forming apparatus prevents damage to the image forming apparatus that results from toner sensor error. The image forming apparatus includes a developer housing positioned opposite to a photosensitive body; a toner box for supplying monocomponent toner to the developer housing; a developer roller for developing an electrostatic latent image on the photosensitive body in the developer housing; a toner transport and supply member for transporting and supplying the monocomponent toner to the developing roller; and a toner sensor for detecting either the presence or the absence of the monocomponent toner in at least one of the developer housing and the toner box. The error detection apparatus includes a first counter for counting the number of either prints or image signals while a toner-present signal is being detected continuously from the toner sensor; a second counter for counting the number of either prints or image signals while a toner-absent signal is being detected continuously from the toner sensor; and a display and stopping unit for stopping the operation of the image forming apparatus and displaying an error indication concurrently if the sum of the values counted by the first and the second counter has exceeded a predetermined value.