An intraoral pressure signal is associated with a pulse wave signal along a time axis. As a calibration coefficient, a ratio of a variation in an amount of change in an amplitude of the pulse wave signal from a preset second reference value to a variation in an amount of change in the intraoral pressure represented by the intraoral pressure signal from a preset first reference value is calculated based on the acquired pulse wave signal and the acquired intraoral pressure signal. An absolute value of an intrathoracic pressure of the subject is calculated by multiplying an estimated intrathoracic pressure, which is a relative value of the intrathoracic pressure estimated based on the pulse wave signal, by the calibration coefficient.
