An agent containing unit for use in an image forming apparatus includes a container, a discharge port unit, and a support plate. The container stores an agent and has an opening on one end portion of the container. The discharge port unit is attached to the opening of the container, and covers the opening of the container and has a first discharge port, through which the agent is discharged from the container. The support plate supports the container, fixed detachably on the support plate, and has a second discharge port, which is aligned with the first discharge port.