A developer supply container detachably mountable to a main assembly of an image forming apparatus, with the image forming apparatus including a remaining amount detecting portion for detecting an amount of remaining developer by receiving light from a light emitting portion by a light receiving portion. The developer supply container includes a container body for containing a developer. The developer supply container also includes a light guiding member including an incident portion for receiving light from the light emitting portion and for guiding the light into the container body, an emergent portion for guiding the light having passed through the container body toward the light receiving portion, and a connecting portion connecting the incident portion and the emergent portion.