PURPOSE: To make it possible to directly confirm the leakage of a primary filter apparatus, by detecting the change in the pressure of a supplied solution caused by the clogging of a micro-filter constituting a secondary filter apparatus.

CONSTITUTION: Because leaked fine particles are inhibited by a micro-filter constituting a secondary filter apparatus, the clogging of the micro-filter is generated and, therefore, pressure loss of the micor-filter becomes high and pressure at the inlet of the micro-filter begins to rise from the pressure during initial operation. Because this pressure change is also generated in a filtrate feed-out pipe 15 for supplying a solution to the secondary filter apparatus 16, said pressure change is detected by a pressure detector 17 provided to the inlet of the secondary filter apparatus or to the exterior of the filtrate feed-out pipe 15 and, when pressure rises above the adjusted point of the pressure detector 17, leakage is informed by issuing an alarm or by lighting an abnormality display lamp.