An ink storage chamber (36) has an indicator formed from magnetic material. A float member (50) whose upper position is limited is housed in the ink storage chamber (36). Two magnetic-field detection system are provided at a position on the exterior of a sub-tank (3), at which the detection system can detect a magnetic flux of the indicator simultaneously, such that the longitudinal direction of the detection system is oriented vertically with a specified ink level of the sub-tank sandwiched between the detection system. On the basis of signals output from the magnetic-field detection system, it is determined whether the ink level is in any one of an excessively low ink level state, a state in which injection of ink must be started, a state in which injection of ink must be stopped, and an excessively supplied state. Thus, an ink level in the sub-tank (3) can be controlled within a specific range without involvement of an undesired increase in the number of sensors.