In order to enhance the performance of the inkjet printer, it is important to be able to appropriately control the ink fluidity in the imaging process. For examples, it is useful to know how the ink fluidity changes when inside the inkjet head or when adhering to the media. As a method that can be used for this judgment, it was attempted to use method of optical coherence tomography was attempted. It is clarified that the fluidity distribution and its time change inside the ink can be visualized by optical measurement and signal analysis. It will bring way to know if the control condition of ink fluidity is appropriate.