Femtosecond dynamics of the photochromic reaction of a thiophene oligomer with a diarylethene structure in solution was analyzed by transient absorption spectroscopy. It was found that the intermediate species with an absorption maximum at similar to 515 nm was formed within 100 fs and that the closed-ring form of the thiophene oligomer was formed mainly from the intermediate with a time constant of similar to 1.1 ps.