To provide a device for reliably measuring differential pressure by preventing power object or the like from being deposited in a monitor pipe for clogging.

An air-feeding pipe 2 is mounted to one portion of a monitor pipe 1 where a tip part is provided in an exhaust pipe 101 and at the same time a differential pressure meter 103 is connected to a base edge part, a first solenoid valve A is provided at one portion of the air-feeding pipe 2, a high- pressure gas supply means for supplying high-pressure air or nitrogen gas to the air-feeding pipe 2 is provided, a second solenoid valve B is provided closer to the differential pressure meter 3 than the mounting site of the air-feeding pipe 2 in the monitor pipe 1, and the operation of the first and second solenoid valves A and B is appropriately controlled for preventing the clogging/deposition of powder or the like in the monitor pipe 1.