Gold nanoparticles covered with photochromic diarylethene polymers or block copolymers with styrene were prepared according to Brust's method. These nanoparticles expressed a bright red color because of their surface plasmon resonance absorption, and exhibited reversible photochromism upon alternate irradiation with ultraviolet and visible light in the solid state as well as in solution. The photocyclization reactivity of the diarylethenes around the gold nanoparticles decreased by shortening the distance between the surface of the gold nanoparticles and the diarylethene chromophore. Some effects of the gold nanoparticles on the optical properties of the diarylethene chromophores were evaluated by comparison of the difference spectra between the UV-visible absorption spectra in the photostationary state and in the initial state under various conditions. The effect was found to appear as a bathochromic shift in the absorption maximum of the diarylethene closed-ring form, which is related to the local electric field generated by the surface plasmon resonance of the gold nanoparticles.