Image forming apparatus

An image forming apparatus comprising:

• an image bearing member;
• a developing unit which is detachably attached to the image forming apparatus and develops an electrostatic latent image formed on the image bearing member, by using a developer including non-magnetic toner and a magnetic carrier;
• a discharge portion which is provided in the developing unit and configured to discharge a developer in the developing unit to the outside of the developing unit;
• a collection portion which collects the developer discharged from the discharge portion;
• a conveying pipe which is separable from the developing unit, includes a conveying screw therein, and conveys the developer discharged from the discharge portion to the collection portion;
• and
• a toner density sensor which detects toner density in the conveying pipe and of which a detection surface is positioned below a rotational axis of the conveying screw in a vertical direction,
• wherein the developing unit is removed from the image forming apparatus in a state in which the conveying pipe and the toner density sensor remain in the image forming apparatus.