This paper presents a principle for a novel electrical molecular memory device using a photochromic diarylethene, which has the potential ability of inexpensive, disposable, extremely high bit-density, and very low power consumption. This device is based on an isomerization reaction of the diarylethene molecule via its excited state by injection of electric carriers, not by photon absorption. The reversible writing by carrier injection is demonstrated. The advantages and applications of the molecular memory are discussed.