To provide a coating fluid for forming an undercoat layer having high stability, a high quality and long-life electrophotographic photoreceptor capable of forming a high quality image in various environments, with which image defects such as black spots or color spots hardly occur, an image forming apparatus using such a photoreceptor, and an electrophotographic cartridge using such a photoreceptor.A coating fluid for forming un undercoat layer of an electrophotographic photoreceptor containing titanium oxide particles and a binder resin, characterized in that titanium oxide agglomerated secondary particles in the coating fluid have a volume average particle size of at most 0.1 μm and a cumulative 90% particle size of at most 0.3 μm.