A blood pressure measuring system includes: a sphygmomanometer which is configured to invasively measure a blood pressure value of a subject a calibrating section which is configured to calibrate a measurement reference value of the blood pressure value a first atmospheric pressure sensor which is adapted to be attached to the subject a second atmospheric pressure sensor which is fixed to a predetermined position a difference acquiring section which is configured to acquire a difference between a first atmospheric pressure value detected by the first atmospheric pressure sensor, and a second atmospheric pressure value detected by the second atmospheric pressure sensor and a determining section which, based on the difference, is configured to determine whether a calibration of the measurement reference value is necessary or not.
