PURPOSE:To detect a failure resulting from the separation of a container and a resin layer in a hollow fiber module uniformly with high accuracy. CONSTITUTION:A slit-like light is irradiated by a light radiating device 2 to a minute area of an interface K between a container A and a resin layer C1. A reflecting light reflected at a separated part in the minute area is measured by a light amount measuring device 3. A separation signal corresponding to the separated area is formed by a separation signal forming part 4 on the basis of the measuring result at the device 3. Whether the hollow fiber module is good or bad is detected in a detecting device 5 based on a plurality of separation signals related to the minute area. In this manner, the separating state between the container A and the resin layer C1 is quantitatively detected by an optical means, whereby the failure resulting from the separation can be automatically and positively detected without irregularities.