A toner bottle cap has a fixed member and a slide member. The fixed member is stationary relative to a toner bottle and defines a first flow passage. The slide member defines a second flow passage coaxially adjoining the first flow passage in a toner outflow direction and having a peripheral wall in sealing contact with a peripheral wall of the fixed member. The slide member is slideable relative to the fixed member in axial direction of the flow passages. One of the members has a wall part that restricts the cross-section of the flow passage of the one member and defines a toner outflow opening. The other of the members has a stop member projecting axially towards the outflow opening and closes the same when the slide member is in a closed position and opens the outflow opening when the slide member is slid in the toner outflow direction.