PURPOSE: To labor-savingly detect the clamping of the filter plates of a horizontal filter plate-type filter by measuring the pressure in a filtration chamber a specified time after a pipeline communicating with the chamber is closed and then pressurized to a specified value.

CONSTITUTION: Filter plates 1 are vertically mounted to form a filtration chamber, and a suspension is filtered by a filter medium inserted between the plates 1 in the horizontal filter plate-type filter. The clogging of the plates 1 is confirmed as described below. A filter medium 4 is firstly inserted between the plates 1, and then the plate stack is clamped by an opening and closing device 6 to form a filtration chamber. Shut-off valves 16 and 16' are then closed to close a raw liq. pipeline and a filtrate pipeline, compressed air is introduced to pressurize the filtration chamber to a specified value, and then a shut-off valve 15 is closed. The pressure in the filtration chamber is measured by a pressure indicator controller 13 after a specified time, and an alarm is issued when the pressure is abnormally high as compared with the pressure drop in normal clamping.