In this paper, we experimentally examine the relationship between visual cognition difficulty and target-tracking eye movements, which recorded during moving target cognition. Generally, such eye movements are observed when humans perceive a moving object and they vary widely due to many factors, such as target shape, backgrounds, illumination conditions, and so on. Several systems have been proposed for estimating human cognition based on gaze movements. However, since most of them employ simple thresholding techniques to classify the states of cognition, their classification performance remain insufficient. This research clarifies the relationship between multiple visual conditions and target-tracking eye movements to enhance the classification performance. We found that we can address a variety of target and background factors from the perspective of visual cognition difficulty for the targets. Observed gaze movement properties and the difficulty of target visual cognition have a linear relationship, suggesting the possibility of more precise estimation of human's target cognition based on them.