PURPOSE: To properly detect a pump drive condition by providing a magnetic sensor for detecting a valve in a casing having a dam of the prescribed cross sectional area on a flow passage and, then, identifying the occurrence of an air hammer, when the sensor does not detect any valve travel.

CONSTITUTION: When the compressed air is fed through an intake port 36, a plunger pump 1 moves a plunger 28 downward via a piston 26, thereby causing a piston 45 to pressurize a viscous fluid in an inlet chamber 44 and feeding the liquid to a delivery chamber 45 via a communication port 43a. Then, when the piston 26 moves upward, the liquid in the chamber 45 passes a delivery port 42 and is fed to a spray nozzle via a detection device 50. The device 50 is in contact with a dam 55, when no fluid flows. Furthermore, the device 50 has a valve element (steel ball) 56 energized with a spring 58 toward the dam 55, and identifies the occurrence of an air hammer, when a magnetic sensor 60 does not detect any travel of the valve 56 at the time of driving the pump 1.