A developing unit includes a unit main body having an internal space defined by a predetermined frame and capable of supplying a developer to the outside while agitating the developer within the internal space, a unit-side cooling duct attached to the unit main body, and a duct connecting member connecting the unit-side cooling duct and an external duct which supplies a cooling gas to the unit-side cooling duct from outside the unit main body. The unit-side cooling duct is structured by a duct piece and a part of the predetermined frame. The duct piece is detachably attached to the unit main body in opposed relation to the part of the predetermined frame, and has an end that is fitted to the duct connecting member.