PURPOSE: To provide a pressure difference detector, which is simple in structure and used for ON-OFF operation or is also applicable to a measure of quantitative pressure difference detection, by detecting the displacement in the cylinder of a piston by a piston displacement detecting means.

CONSTITUTION: A detector body 11 has a hollow cylinder 14, and inside the cylinder 14 is a piston 17 incorporated in axial direction of the cylinder. And, when the pressure falling on the top of the piston 17 is equal to that on the bottom, the piston 17 exists in the middle position of a piston support spring 15, but if a difference occurs in the pressure, it shifts to the position where the resultant accompanying the pressure difference and the spring force of a spring 15 balance each other. When the piston 17 shifts upward by the pressure difference, the electric contact 29 on piston side and the electric contact 27 on cylinder side contact with each other. These contacts 27 and 29 are connected to a piston displacement detecting circuit 23, and the circuit 23 changes the contact signals generated at the contacts 27 and 29 into voltage signals, and the output signals are sent to a controller 25, and are used for occurrence and indication of an alarm.