To provide a system for measuring an extending width of an emulsion layer in a desalting vessel with a high accuracy and at real time.

In the desalting vessel 4 in which the emulsion layer 14 being formed by introducing a crude oil having been injected with an emulsion breaker and a water and having been stirred, and further the oil and the water being separated into an upper and a lower zones of the vessel interrupted by said emulsion layer, wherein a saline matter being separated from said crude oil by being dissolved in said water, and thereby the desalted oil being discharged from the top of the vessel and the separated water being discharged from the bottom of the vessel, the extending width of said emulsion layer is measured on the basis of the distribution of the pressure difference between the pressure of the inner solution ranging from the top to the bottom of the vessel including said emulsion layer and the pressure of the discharged water.