The internal structure of a novolak/quinonediazide type positive photoresist image was observed using a cross‐sectional exposure method. On the surface of a photoresist image, a photochemically inert layer was detected which does not return to diazo and phenolic compounds with acid treatment. This shows that this layer does not consist of the azoxy compounds. Fourier transform infrared spectrum indicates that this layer includes azocoupling reaction products. It was also found that the formation of the photochemically inert layer and the dissolution of the layer takes place simultaneously at the development front and a resist image is formed during the competition between these two reactions.