Working memory is a system for processing and storing information. The Reading Span Test (RST), developed by Daneman and Carpenter (1980), is well-known for assessing individual difference in working memory. In the present investigation, we used the Japanese version of the RST (Osaka, 2002) and analyzed individual differences in strategy use from the viewpoint of strategy type (rehearsal, chaining, word-image, scene-image, and initial letter) and frequency of use (used in almost all trials, in half the trials, or not used). Data from the participants (N = 132) were assigned to groups according to the scores, for the total number of words correctly recalled and the proportion correct. The results showed that the frequency of word-image strategy use differed significantly between high-scoring subjects (HSS) and low-scoring subjects (LSS). HSS mainly used word-image and chaining strategies, while LSS used rehearsal and chaining strategies. This indicates that HSS used both verbal and visual strategies, whereas LSS relied only on verbal strategies. The use of the word-image is important for effective retention of words in memory.