PURPOSE: To check the passage of a packing agent in a separation column through a filter, by forming a layer of particulate material with the size larger than a particle size of the packing agent not to pass through a porous filter between a layer of the packing agent and the porous filter.

CONSTITUTION: Between a packing agent 2 and a filter 4, a particulate material 3 similar to the packing agent 2 by nature is packed externally above and below a layer of the packing agent 2 in several levels of layers. When an eluate is fed to a separation column 1 from a feed liquid pump, outflow of particles of the packing agent blocked by the layers of the particulate material 3. When there is a hole clearance larger than the particle size of the packing agent 2 in the filter 4, the particulate material enters thereinto thereby eliminating a relatively large hole in substance. Thus, passage through the filter 4 is checked completely even if the particles of the packing agent 2 pass through the layers of the particulate material.