A novel all-optical parallel switching device was demonstrated by using photoinduced changes of a complex refractive index in a guided mode thin film composed of silver and polymer containing photochromic dyes. An incident angle of a probe beam was set at a value corresponding to the minimum reflection due to the guided mode. Pulsed laser excitation caused changes of absorption of the polymer films due to photochromism, which resulted in the increase of reflection because the guided mode condition was changed. According to the Kramers-Kronig relationship the real part also changes, which results in the shift of the minimum. Appropriate wavelength can be selected for all-optical switching in this geometry. We used photochromic spiropyran dispersed in polystyrene. Self-held parallel switching was achieved upon UV or visible laser excitation with response times less than 20 ns.