To provide a apparatus for measuring differential pressure which can measure a differential pressure simply, without requiring laying of pipelines.

The differential pressure measuring apparatus which measures the differential pressure with a reference pressure and which can be carried, includes a first air chamber 2, a second air chamber 3, bellows 5 telescoped and contracted by the differential pressure of these first air chamber 2 and second air chamber 3, and a displacement detector 6 which detects the displacement of these bellows 5. When the differential pressure is measured, the body of the apparatus is carried in the part used as the reference pressure, communication of the first air chamber 2 is performed in the part; and this first air chamber 2 is sealed. In this state, the body of the apparatus is carried in the part for measuring the differential pressure and is configured with the second air chamber 3 communicating with the part for measuring the differential pressure.