A method for correcting an image in a printing apparatus including line heads, the method comprising: moving first and second ink jet heads and a medium (m) relative to each other, the first and second ink jet heads being arranged along a first direction, each of the first and second ink jet heads including nozzles and drive elements corresponding to the nozzles respectively, the first and second ink jet heads and the medium (m) being moved relative to each other in a second direction intersecting with the first direction; discharging ink droplets from the nozzles of the first and second ink jet heads, by connecting each of the drive elements to one of power supply circuits selected from among the power supply circuits and by applying voltage to each of the drive elements at a predetermined timing while moving the first and second ink jet heads and the medium (m) in the second direction, wherein the power supply circuits are all set so that their output voltages differ; determining a density difference between a first image region and a second image region, the first image region being formed on the medium (m) by the ink droplets discharged from first nozzles, the first nozzles being included in the nozzles of the first ink jet head and being positioned in an end portion on one side in the first direction among the nozzles of the first ink jet head, the second image region being formed on the medium (m) by the ink droplets discharged from second nozzles, the second nozzles being included in the nozzles of the second ink jet head and being positioned in an end portion on the other side in the first direction among the nozzles of the second ink jet head, and wherein the end portions of the first ink jet head and the second ink jet head overlap in the second direction, and a number of nozzles positioned in the end portion of the first ink jet head and a number of nozzles positioned in the end portion of second ink jet head are configured as non-discharging nozzles; and switching the voltage to be applied to first drive elements to a first voltage by switching the power supply circuits to be connected to the first drive elements based on the density difference, the first drive elements being included in the drive elements of the first ink jet head and corresponding to the first nozzles, the first voltage being different from the voltage to be applied to drive elements, of the first ink jet head, other than the first drive elements, wherein the switching the voltage to be applied to first drive elements is by switching the power supply circuits to an unused power supply circuit that has had its output voltage changed, such that the density difference is made smaller. moving first and second ink jet heads and a medium (m) relative to each other, the first and second ink jet heads being arranged along a first direction, each of the first and second ink jet heads including nozzles and drive elements corresponding to the nozzles respectively, the first and second ink jet heads and the medium (m) being moved relative to each other in a second direction intersecting with the first direction; discharging ink droplets from the nozzles of the first and second ink jet heads, by connecting each of the drive elements to one of power supply circuits selected from among the power supply circuits and by applying voltage to each of the drive elements at a predetermined timing while moving the first and second ink jet heads and the medium (m) in the second direction, wherein the power supply circuits are all set so that their output voltages differ; determining a density difference between a first image region and a second image region, the first image region being formed on the medium (m) by the ink droplets discharged from first nozzles, the first nozzles being included in the nozzles of the first ink jet head and being positioned in an end portion on one side in the first direction among the nozzles of the first ink jet head, the second image region being formed on the medium (m) by the ink droplets discharged from second nozzles, the second nozzles being included in the nozzles of the second ink jet head and being positioned in an end portion on the other side in the first direction among the nozzles of the second ink jet head, and wherein the end portions of the first ink jet head and the second ink jet head overlap in the second direction, and a number of nozzles positioned in the end portion of the first ink jet head and a number of nozzles positioned in the end portion of second ink jet head are configured as non-discharging nozzles; and switching the voltage to be applied to first drive elements to a first voltage by switching the power supply circuits to be connected to the first drive elements based on the density difference, the first drive elements being included in the drive elements of the first ink jet head and corresponding to the first nozzles, the first voltage being different from the voltage to be applied to drive elements, of the first ink jet head, other than the first drive elements, wherein the switching the voltage to be applied to first drive elements is by switching the power supply circuits to an unused power supply circuit that has had its output voltage changed, such that the density difference is made smaller.