A developer accommodating device, detachably mounted on a developing unit, includes a frame extending in a longitudinal direction, and configured to accommodate toner and discharge the toner through an opening portion, a guide portion configured to be guided by the developing unit while the developer accommodating device is being mounted on the developing unit in a mounting direction crossing the longitudinal direction, and a first portion including a first chamber, in which the opening portion is provided, and a second chamber which is aligned in a first direction perpendicular to the longitudinal direction. The first chamber of the first portion communicates with the second chamber of the first portion only through a second portion.