The micro-Brownian motion of a single polymer chain was directly measured by high-speed (rapid-scan) AFM on a mica substrate under an aqueous solution at room temperature. The diffusion constant was 1.3 x 10(-14) m(2)/s, which is a basic property of a molecular bearing at a solid-liquid interface. The molecule was a micrometer-long supramolecular chain in which pi-conjugated polyrotaxane was coupled by a pi-pi interaction. Bond scission of this supramolecular chain was directly observed, and the dynamic folding behavior of a single polymer chain was also imaged.