The molecular wire approach has recently been proposed as a method to enhance the sensitivity of traditional chemosensors. In this paper, we propose a simple model of a molecular wire sensor (MWS) that explains experimentally observed non-ideal behaviors of the MWS, whose signal intensity decreases by a constant factor g per each binding of an analyte molecule to a molecular wire. Quantitative relationship between analyte density and the signal intensity of the MWS is established. We find that the sensitivity of the MWS to the change of analyte density increases with the number k of receptor units in each molecular wire composing the MWS if k is less than a critical value k*; otherwise it becomes a decreasing function of k. It turns out that signal patterns of the MWS differ from those of traditional sensors; the Stern-Volmer curve of the MWS has a positive or negative curvature depending on the analyte density and physical characteristics, such as g and k, of the MWS.