This study aims to propose a comprehensive approach that utilizes both linguistic information and illustrations (visual images) created by intermediate- to advanced-level learners of Japanese and to examine the complementary role of visual images in interlanguage. First, by using a selection of Haruki Murakami’s novels as material, we identified characteristics of interlanguage such as permeability, transitionality, and fossilization in the learners’ illustrations. We argued that visual images also form an integral part of the interlanguage system. Next, introducing a novel approach to examining the universality of interlanguage from a quantitative perspective, using a folktale as material, we analyzed the illustrations drawn by learners as continuous-type distribution data, based on the average evaluation scores of native Japanese speakers. We concluded that learners’ images evolve similarly to linguistic items and approach the target language incrementally, with each stage intermingled with common features.