An ink supply mechanism (50) provided with: a main tank (60); a sub-tank (70) for storing ink supplied from the main tank (60) and supplying the ink to an ink jet head (2); a first communication passage (52) one end of which communicates into the ink in the main tank (60) and the other end of which communicates into the ink in the sub-tank (70); and a second communication passage (53) one end of which communicates into the main tank (60) and the other end of which communicates with a position in the sub-tank (70) higher than the other end of the first communication passage (52), and part of which is disposed at a position higher than the liquid level of the ink in the main tank (60).