To provide a lung compliance estimation apparatus and estimation method capable of accurately obtaining the compliance of lungs.

An airway flow rate QawO at the point T0 of time of starting support and airway resistance R estimated beforehand are multiplied and an intrapulmonary pressure PalvO (=RQaw0) at the point T0 of time of starting the support is computed. Also, the computed intrapulmonary pressure Palv0 at the point T0 of time of starting the support is subtracted from the airway pressure Paw2 at the point T2 of time of an occlusion end stage and a pressure deviation (Paw2-Palv0) is computed. On the basis of the pressure deviation (Paw2-Palv0), computation is carried out as the compliance of lungs of a patient. Immediately after the inspiration operation of the patient is started, a spontaneous breathing pressure Pmus indicating the breathing effort of the patient is extremely low. By using a detected result in such a state that the spontaneous breathing pressure Pmus is extremely low, the compliance of the lungs is accurately estimated.