A developing cartridge is detachably attached to an electrophotograpy-type image forming apparatus which forms an image on a recording sheet by transferring toner onto the recording sheet. The developing cartridge includes a casing that provides a toner accommodating space for accommodating toner, and an agitator that is rotatably disposed in the casing and agitates toner accommodated in the toner accommodating space. The developing cartridge has a light guiding member that is disposed in the casing go as to be rotatable integrally with the agitator. The light guiding member includes a light entrance portion disposed at a position offset from a rotation shaft of the agitator and allowing entrance of light into the light guiding member, and guides the light entering from the light entrance portion to an outside of the casing.