Abstract: A reference level detection system for detecting a reference level of a document comprises a background level detection circuit supplied with an input signal indicative of a background level of the document for producing a first output signal which follows a change of the background level of the document, digital feedback loop supplied with the first output signal for producing a digital data such that the digital data is changed in correspondence to the first output signal, the foregoing digital feedback loop further including a circuit for producing a second output signal from the digital data and supplied further with a command signal for prohibiting a change of the digital data at least in one direction, and a switch supplied with the command signal for selectively providing either one of the first and second output signals as the reference level such that the first output signal is provided as the reference level when the command signal is in a first state not prohibiting the change of the digital data a