To properly detect a reference pressure for controlling a room pressure.

A fine differential pressure transmitter includes: a differential pressure detection part for detecting a difference between pressure used as a reference and pressure of a measurement target; a first pressure guide part for guiding the pressure used as a reference to the differential pressure detection part; and a second pressure guide pipe for guiding the pressure of the measurement target to the differential pressure detection part. The first pressure guide part includes: an opening for introducing pressure used at least as a reference; a bag body for hermetically sealing and covering the opening; and a box body surrounding the bag body and having a communication part for communicating with the inside and the outside of the bag body.