To prevent the damage of a capillary tube by composing a high pressure side or low pressure side pressure transmitting means of the coil-like capillary tube and an expandable, strong protective pipe covering the capillary tube.

A differential transmitter 1 has a differential transmitter body part 2, a high pressure side seal diaphragm part 5A, a low pressure side seal diaphragm part 5B and two sealing liquid filling pressure transmitting means. The body part 2 detects differential pressure between a high pressure side measuring chamber 3A and a low pressure side measuring chamber 3B and converts it into a transmission signal. The high pressure side pressure transmitting means is composed of a coil-like capillary tube 8 and an expandable protective pipe covering the capillary tube 8 completely. This protective pipe is composed of strong hollow pipes 9, 10, 11 made of metal. The low pressure side pressure transmitting means is composed of a connecting member 15 with a communicating hole 16 bored along its center axis. Work for fitting the body part 2 and two seal diaphragm parts 5A, 5B to a measuring pipe 20 is therefore simplified.