To provide a method capable of evaluating photocatalyst function with a simple facility regardless of the material of a base material, and a device suitable for executing this method.

In this method comprising coloring the surface of a photocatalyst function film layer 2 formed on a base material 1, emitting ultraviolet ray to the colored surface, and measuring the absorbance or reflective index thereof to evaluate the function of the photocatalyst, the ultraviolet ray is continuously emitted during the measurement time, and the photocatalyst function is evaluated by the change of the value of transmittance or absorbance measured by emitting the measuring light during the measurement time. Each pair of cases for attachably and detachably housing a light emitting element or light receiving element, which has a base having a through-hole in the placing part of the base material 1 having the photocatalyst function film layer 2 with colored surface, and a black light lamp arranged in the front of the surface of the base material 1, is provided on the axial line passing the base material 1 opposite to each other in the longitudinal direction of the base material 1, and also on the reflected light axis in the front of the base material 1, respectively.