A buffer tank mounted on a head holder has a plurality of air discharge valves for discharging the air accumulated in inks to the outside. The air discharge valve includes a valve body for opening and closing the passage bore and a valve rod provided extending from the valve body downwardly. A maintenance unit is provided outside the buffer tank, which includes an air discharge cap for covering the lower ends of the passage bores and a plurality of air discharge rods provided corresponding to the air discharge valves respectively. The air discharge rods are different in length from one another, depending on the colors of ink. When the air discharge rods are lifted up simultaneously by a lifting, they come into direct contact with the lower ends of the corresponding valve rods in sequence as their movements are delayed one after another, hence opening the valve bodies separately.