A developer container (30) includes a container body (31), a movable wall (34), and a backward movement preventing mechanism (55). The container body (31) includes an inner surface (31K) defining a cylindrical internal space (31H) extending in a longitudinal direction, and a developer discharge port (319). The movable wall (34) includes an outer surface (34K) disposed slidably in close contact with the inner surface (31K) of the container body (31), and a conveying surface (340S). The conveying surface (340S) defines a storage space (31S) for developer. The movable wall (34) moves in the longitudinal direction from an initial position at one end side to the developer discharge port (319) while conveying the developer in the storage space (31S) to the developer discharge port (319). The backward movement preventing mechanism (55) prevents the movable wall (34) at the developer discharge port (319) from moving back toward the initial position.