Design sketches are believed to play essential roles in early conceptual design processes. Exploration of how sketches are essential for the formation of new design ideas is expected to bring important implications for design education and design support systems. Little research has been done, however, to empirically examine the ways in which designers cognitively interact with their own sketches. Using a protocol analysis technique, we examined the design thoughts of an architect from the following point of view; how he drew depictions, inspected depicted elements, perceived visuo-spatial features, and thought of non-visual functional or conceptual information. The findings suggest that design sketches serve not only as external memory or as a provider of visual cues for association of non-visual information, but also as a physical setting in which design thoughts are constructed on the fly.