In a detection unit, a measurement electrode member is arranged at a position where it contacts a developer, and has an input electrode for receiving a voltage from an apparatus main body and an output electrode for outputting an electrical signal in accordance with an electrostatic capacitance generated upon application of the voltage. A reference electrode member is arranged at a position where it does not contact the developer, and has an input electrode for receiving a voltage from the apparatus main body and an output electrode for outputting an electrical signal in accordance with an electrostatic capacitance generated upon application of the voltage. The measurement electrode member is adhered to an adhesion member. A lid member is coupled to the adhesion member and covers the reference electrode member. An engaging portion engages with a vessel positioning portion attached to the developer vessel, and positions the detection unit with respect to the developer vessel. A developer vessel, process cartridge, developing cartridge, and electrophotographic image forming apparatus are also disclosed.