The present study investigated the relationship between working memory capacity and brain activity in the dorsolateral prefrontal cortex (DLPFC) and the left ventrolateral prefrontal cortex (VLPFC) using functional near-infrared spectroscopy. We administered the reading span test language task to measure linguistic working memory, while we assessed brain activity reflected in the cerebral blood flow change during the task. The cerebral blood flow change in the DLPFC had a negative correlation with the reading span test score. Both focused regions exhibited different patterns of cerebral blood flow changes during the first and second halves of the task period in the high-and low-score groups. Our findings suggest that the brain activity correlates with the reading span test score, which is associated with linguistic working memory capacity. Therefore, brain activity measurement in the DLPFC with the functional near-infrared spectroscopy can be used to indirectly assess working memory capacity.