To provide a pressure sensor for eliminating the need for providing a seal diaphragm especially, and a differential pressure transmitter that has the pressure sensor and a simple structure, can be miniaturized, and has a low cost.

A pressure sensor consists of a substrate 2, a diaphragm 3 where a semiconductor strain gauge 10 is formed in the substrate 2, an upper thick part 4 that is located at the upper portion of the diaphragm 3 in the substrate 2, a part for fixing the diaphragm 3 and the upper thick part 4 on the substrate 2, and a seal diaphragm 6 for separating the diaphragm 3, the upper thick part 4, and a fixing part 5 from a fluid to be measured at the upper portion of the substrate 2. A sealed liquid 8 is filled into an inner gap 7. Also, the surface of the seal diaphragm 6 is coated with a corrosion-resistance thin film 9, thus eliminating the need for an external seal diaphragm since the pressure sensor itself has the seal diaphragm 6 and is coated with a corrosion-resistance thin film 9 and hence simplifying and miniaturizing a differential pressure transmitter and reducing its cost.