A system of detecting an ink use amount of an inkjet printer configured to perform image forming by discharging an inkjet ink on a web-shaped print base material by a single-pass method, the system of detecting an ink use amount comprising: a main ink tank for storing an inkjet ink to be discharged from an inkjet head; a base which is movable, and the main ink tank is being placed on the base; an ink tank mass measuring device configured to measure a mass of the main ink tank; a comparison operation device configured to perform a comparison operation between a setting reference value of an ink use amount, which is prepared in advance, and a use amount measurement value of the ink tank mass measuring device; a display device configured to display a warning when a difference between the use amount measurement value and the setting reference value of the ink use amount exceeds a range set in advance; a sub-tank which is provided so as to correspond to the main ink tank; a sub-tank liquid level sensor which detects an ink amount of the inkjet ink of the sub-tank; a supply pump which supplies the ink jet ink in the main ink tank to the sub-tank when the sub-tank liquid level sensor detects that the ink liquid surface in the sub-tank is lower than a predetermined position; wherein the setting reference value of the ink use amount is a calculated value by the comparison operation device based on an ink volume or an ink mass using mass data of a discharge ink droplet after converting received image data into discharge data for printing; wherein, as a result of the comparison operation in the comparison operation device, when the difference between the use amount measurement value and the setting reference value of the ink use amount is within a range set in advance, the printing is continued as it is; and wherein when the difference between those two values exceeds the range set in advance, the printing is stopped as operation abnormality, and an alarm is displayed on the display device. a main ink tank for storing an inkjet ink to be discharged from an inkjet head; a base which is movable, and the main ink tank is being placed on the base; an ink tank mass measuring device configured to measure a mass of the main ink tank; a comparison operation device configured to perform a comparison operation between a setting reference value of an ink use amount, which is prepared in advance, and a use amount measurement value of the ink tank mass measuring device; a display device configured to display a warning when a difference between the use amount measurement value and the setting reference value of the ink use amount exceeds a range set in advance; a sub-tank which is provided so as to correspond to the main ink tank; a sub-tank liquid level sensor which detects an ink amount of the inkjet ink of the sub-tank; a supply pump which supplies the ink jet ink in the main ink tank to the sub-tank when the sub-tank liquid level sensor detects that the ink liquid surface in the sub-tank is lower than a predetermined position; wherein the setting reference value of the ink use amount is a calculated value by the comparison operation device based on an ink volume or an ink mass using mass data of a discharge ink droplet after converting received image data into discharge data for printing; wherein, as a result of the comparison operation in the comparison operation device, when the difference between the use amount measurement value and the setting reference value of the ink use amount is within a range set in advance, the printing is continued as it is; and wherein when the difference between those two values exceeds the range set in advance, the printing is stopped as operation abnormality, and an alarm is displayed on the display device.