A developing agent leakage preventing member is provided that is attached to the developer housing and that has a housing attaching face which contacts with the developer housing, a front side face which opposes the housing attaching face, and two side faces each of which connects to each other end portions of the housing attaching face and the front side face in the longitudinal direction of the opening of the developer housing. At least one of the side faces of the developing agent leakage preventing member is formed as an inclined side face that is inclined in a predetermined direction relative to the housing attaching face when pressed from the front side face side. In a developing apparatus, developing agent (toner) leakage is avoided.