To provide a pressure detector by which the pressure difference between a pressure applied to a liquid and a pressure applied to a gas can be detected easily by a method wherein the pressure difference between an intake pressure and a fuel pressure can be detected by eliminating the difficulty that a driver cannot measure simultaneously, during a running operation, both an intake pressure value and a fuel pressure value which are changed with dizzily rapidity according to a driving operation.

The pressure detector is provided with a fuel-pressure introduction part 7 in which the pressure of a fuel supplied by a fuel pump C is introduced to an internal-combustion engine A mounted on a vehicle, an intake- pressure introduction part 15 in which the intake pressure of the air supplied to the engine A is introduced and a pressure detection means 1 which detects the pressure difference between the pressure of the fuel transmitted from the part 7 and the intake pressure transmitted from the part 15.