To provide a differential pressure measuring instrument without variations with the lapse of time, in which there is no need for repair or physical calibration even when immersed in a shield liquid.

The differential pressure measuring instrument is vertically provided from the bottom surface 14A so that the upper surface is above the liquid surface 12A of the shield liquid 12 contained in a shield liquid containing part 14. In the differential pressure measuring instrument, which includes a communication tube 24 for making upper and lower spaces divided into two by the bottom surface 14A into communication with each other, a sunk bell 26 which is formed like a tube having a lid having an opening at a lower part and which shuts off the communication state of the first space 18 in communication with a first external communication opening 16 and the second space 22 in communication with a second external communication opening 20, by immersing the opening in the shield liquid 12, and a sunk bell movement measuring part 30 for measuring the vertical displacement of the sunk bell 26 moved according to the differential pressure between the first and second spaces 18 and 20, the sunk bell movement measuring part 30 is a laser displacement measuring instrument 38 for measuring the displacement of the sunk bell 26 by irradiating the upper surface of the sunk bell 26 with a laser.