A developer container, comprising: a container body including an inner surface defining a cylindrical internal space extending in a longitudinal direction, and a storage space provided in the internal space for containing developer, the container body being formed with a developer discharge port in a lower part of the inner surface and communicating with the storage space for discharging developer therethrough; a movable wall disposed in the internal space of the container body, and including an outer surface disposed slidably in close contact with the inner surface, and a conveying surface defining the storage space in cooperation with the inner surface of the container body, the movable wall being movable in the longitudinal direction from an initial position set at one end side to the developer discharge port lying at the other end side of the internal space to seal the developer discharge port by the outer surface, while conveying the developer in the storage space to the developer discharge port; and a backward movement preventing mechanism configured to prevent the movable wall at the developer discharge port from moving back toward the initial position, the movable wall includes a conveying wall portion having the conveying surface, and an outer peripheral wall portion extending from an outer peripheral edge of the conveying wall portion to an upstream side in a moving direction of the movable wall characterized in that the outer peripheral wall portion has ribs disposed on the outer peripheral wall portion and extend in the longitudinal direction, the ribs being spaced from one another in a circumferential direction of the outer peripheral wall portion, the ribs are in contact with the inner surface of the container body and function to prevent the movable wall from tilting in the longitudinal direction in the container body. a container body including an inner surface defining a cylindrical internal space extending in a longitudinal direction, and a storage space provided in the internal space for containing developer, the container body being formed with a developer discharge port in a lower part of the inner surface and communicating with the storage space for discharging developer therethrough; a movable wall disposed in the internal space of the container body, and including an outer surface disposed slidably in close contact with the inner surface, and a conveying surface defining the storage space in cooperation with the inner surface of the container body, the movable wall being movable in the longitudinal direction from an initial position set at one end side to the developer discharge port lying at the other end side of the internal space to seal the developer discharge port by the outer surface, while conveying the developer in the storage space to the developer discharge port; and a backward movement preventing mechanism configured to prevent the movable wall at the developer discharge port from moving back toward the initial position, the movable wall includes a conveying wall portion having the conveying surface, and an outer peripheral wall portion extending from an outer peripheral edge of the conveying wall portion to an upstream side in a moving direction of the movable wall characterized in that the outer peripheral wall portion has ribs disposed on the outer peripheral wall portion and extend in the longitudinal direction, the ribs being spaced from one another in a circumferential direction of the outer peripheral wall portion, the ribs are in contact with the inner surface of the container body and function to prevent the movable wall from tilting in the longitudinal direction in the container body.