The present invention provides a memory device and a semiconductor device which have high reliability for writing at low cost. Furthermore, the present invention provides a memory device and a semiconductor device having a non-volatile memory element in which data can be additionally written and which can prevent forgery due to rewriting and the like. The memory element includes a first conductive layer, a second conductive layer, and an organic compound layer, which is formed between the first conductive layer and the second conductive layer, and which has a photosensitized oxidation reduction agent which can be an excited state by recombination energy of electrons and holes and a substance which can react with the photosensitized oxidation reduction agent.