A pressure change measuring apparatus for detecting a time variation of a pressure to be measured, comprising: a differential pressure sensor, which has a cavity having a predetermined volume and a communication hole configured to allow a pressure transmission medium for transmitting the pressure to be measured to flow into and out of the cavity, and is configured to output an output signal relating to a differential pressure between an inner pressure (Pin) of the cavity and the pressure to be measured (Pout); and an arithmetic processing unit configured to calculate the time variation of the pressure to be measured, wherein the arithmetic processing unit comprises: a differential pressure calculation unit configured to obtain the pressure difference between the inner pressure of the cavity and the pressure to be measured based on the output signal of the differential pressure sensor; characterized in that the arithmetic processing unit further comprises: a pressure-to-be-measured calculating unit configured to calculate the pressure to be measured based on a set inner pressure of the cavity and the pressure difference calculated by the differential pressure calculation unit; a flow rate calculating unit configured to calculate a flowing quantity of the pressure transmission medium flowing into and out of the cavity for every unit of a predetermined time period based on the pressure difference calculated by the differential pressure calculation unit; and an inner pressure updating unit configured to calculate the inner pressure of the cavity after the predetermined time period based on the flowing quantity calculated by the flow rate calculating unit and the predetermined volume of the cavity, and to update, to the calculated inner pressure, the set value of the inner pressure of the cavity to be used by the pressure-to-be-measured calculating unit. a differential pressure sensor, which has a cavity having a predetermined volume and a communication hole configured to allow a pressure transmission medium for transmitting the pressure to be measured to flow into and out of the cavity, and is configured to output an output signal relating to a differential pressure between an inner pressure (Pin) of the cavity and the pressure to be measured (Pout); and an arithmetic processing unit configured to calculate the time variation of the pressure to be measured, wherein the arithmetic processing unit comprises: a differential pressure calculation unit configured to obtain the pressure difference between the inner pressure of the cavity and the pressure to be measured based on the output signal of the differential pressure sensor; a pressure-to-be-measured calculating unit configured to calculate the pressure to be measured based on a set inner pressure of the cavity and the pressure difference calculated by the differential pressure calculation unit; a flow rate calculating unit configured to calculate a flowing quantity of the pressure transmission medium flowing into and out of the cavity for every unit of a predetermined time period based on the pressure difference calculated by the differential pressure calculation unit; and an inner pressure updating unit configured to calculate the inner pressure of the cavity after the predetermined time period based on the flowing quantity calculated by the flow rate calculating unit and the predetermined volume of the cavity, and to update, to the calculated inner pressure, the set value of the inner pressure of the cavity to be used by the pressure-to-be-measured calculating unit.