Developing device, and image forming apparatus and process cartridge incorporating same

A developing device comprising:

• a casing to contain developer and define a developer circulation passage;
• a developing roller to bear the developer;
• and
• at least one developer conveyor to convey the developer and supply the developer to the developing roller, the at least one developer conveyor disposed in the developer circulation passage and lower than the developing roller,
• wherein the developing device satisfies:
• M1/L1>0.56 g/cm;
• and
• M1/M2>0.50
• where M1 represents an amount of developer borne on the developing roller, L1 represents a width in which the developing roller bears the developer in a longitudinal direction of the developing roller;
• and M2 represents an amount of developer stored in the casing and excluding the amount of developer borne.