PURPOSE: To simplify the measurement of differential pressure by providing a first slide position or the like for connecting a differential pressure port to a high pressure introduction port, a low pressure introduction port, the differential pressure port and a drain port.

CONSTITUTION: A differential pressure detection valve is made up of a valve box 1, a high pressure introduction ports 2 and 3 formed therein 1 and differential pressure port 4 and a drain port 5 therein 1. A gage 15 or other devices to be operated by a differential pressure is connected to the differential pressure port 4 while the drain port 5 is opened to a tank or the atmosphere. A spool 6 slides at the first slide position for connecting the differential pressure port 4 to the high pressure introduction port 2 and at the second position for connecting the differential pressure port 4 to the drain port 5. This enables the measurement of differential pressure with only one gage thereby simplifying it.