A toner density sensor includes: a light emitting element configured to radiate light; a light receiving element configured to receive the light emitted from the light emitting element and reflected from a detection object; a substrate whereon the light emitting element and the light receiving element are surface mounted; a case covering the light emitting element and the light receiving element; an optical path configured parallel to the substrate for light generated from the light emitting element; a groove provided in the substrate from the end part of the substrate in the direction light travels from the light emitting element to between the light emitting element and the light receiving element, the groove recessed from the mounting surface in the thickness direction of the substrate; and an insert provided to the case and configured for insertion into the groove.

A single-layer electrophotographic photosensitive member includes a conductive substrate and a photosensitive layer. The conductive substrate contains aluminum or an aluminum alloy. A surface of the conductive substrate has an aluminum oxide film or an aluminum alloy oxide film. The photosensitive layer is disposed directly on the conductive substrate. The photosensitive layer contains an electron transport material. The electron transport material has a reduction potential of at least 0.88 V and no greater than 0.66 V versus a reference electrode (Ag/Ag.sup.+). The single-layer electrophotographic photosensitive member has a leakage onset voltage of at least 5.0 kV in a high temperature and humidity environment at a temperature of 30 C. and a relative humidity of 80%. The leakage onset voltage is a voltage applied to the single-layer electrophotographic photosensitive member at which current leakage starts.