To provide a flange-mounting type differential pressure measuring apparatus having an enhanced temperature characteristic.

The flange-mounting type differential pressure measuring apparatus includes a pressure sensor provided in a body, a high pressure diaphragm chamber provided in a high pressure side flange to communicate with the high pressure side of the pressure sensor, a low pressure side diaphragm chamber provided on the side face of the body to communicate with the low pressure side of the pressure sensor, and first and second sealed liquids sealed respectively in the high pressure side and the low pressure side of the pressure sensor. The measuring apparatus also includes a compensating sealed liquid reservoir provided in a portion where its temperature is equal to the temperature of fluid measured on the high pressure side, for compensating temperature errors based on the temperature of the fluid measured at the high pressure side, and a communicating pipe for communicating the reservoir with the low pressure side of the pressure sensor.