To provide the heat sensing unit of a differential distribution type heat sensor for simplifying a structure, easily performing test adjustment and being arranged in concentration at one part.

Both ends of an air pipe 1A laid in a guard area are airtightly and freely attachably and detachably connected through the packing 21 of rubber or the like to a connection part main body 22. A pressure sensor 30 is connected through a through-hole 23 provided through the main body 22, pressure signals detected in the pressure sensor 30 are converted to temperature change signals in a signal processing part 40 and the generation of a fire is detected. A test hole 24 provided on the main body 22 is provided with an injection port 25 for connecting a test jig and a measurement port 26. Thus, air for testing is injected while monitoring an air pressure inside the air pipe 1A by using the test jig and the test adjustment of this heat sensing unit 1C is performed. The need of a conventional large-sized cock part provided with a large diaphragm and a leak hole is eliminated, miniaturization is performed and arrangement in concentration is made possible.