Image forming apparatus with gas flowing from developing device to supply device through developer channel

An image forming apparatus comprising:

• an image carrier;
• a developing device configured to cause a developer to adhere to the image carrier;
• and
• a supply device having a developer channel through which the developer passes, and configured to supply the developer to the developing device, the developer channel comprising a filled portion that is filled with the developer over an entire cross section of the developer channel, and a falling portion where the developer falls downward, the falling portion being positioned on a downstream side of the filled portion in a developer movement direction, the developer channel moving developer from the filled portion to the falling portion, the developer channel configured to allow the developer to move only one way from the filled portion to the falling portion, wherein:
• gas flowing from the developing device to the supply device moves upstream in the developer movement direction through the developer channel, and
• a sectional area of the falling portion is larger than a sectional area of the filled portion.