To reduce the number of portions having the possibility of leakage in a tank, to reduce an insatallation cost, an instrumentation cost and a cost of a transmitter, and to enhance maintainability.

In this differential pressure measuring instrument for measuring a liquid level and density of a measured fluid filled in the tank, two low-pressure side pressures are introduced into a differential pressure measuring instrument body using one pressure point, as a reference, in a high pressure side, so as to measure the liquid level and the density based on two differential pressure signals, using the pressure in the high pressure side as the reference.