To exactly measure density of liquid such as muddy water containing solid particles in soil at an original position in the soil.

A differential pressure measurement apparatus 1 of a device S includes: a differential pressure measurement part 10 which has pressure receiving parts 11, 12 by being mutually approximated at upper and lower parts; and a communication tube 20 having an opening 21 at the upper end, which is water-tightly raised on the upper pressure receiving part 11 of the differential pressure measurement part 10 to extend to a predetermined height. The differential pressure measurement apparatus directly brings the lower pressure receiving part 12 into contact with muddy water and the like at a part under the muddy water and the like to receive pressure by the lower pressure receiving part 12, fills water in the communication tube 20 to open the opening 21 on the upper end of the communication tube 20 in the muddy water and the like, brings the opening 21 into contact with the muddy water and the like at a part over the muddy water and the like to receive the pressure by the upper pressure receiving part 11, and measures differential pressure between two parts.