Inclusion Criteria:  Patients ≧ 18 years old Patients with moderate to severe ARDS under mechanical ventilation＊ ＊ Definition of moderate to severe ARDS is as per the Berlin definition (PaO2/FiO2 ≦ 200 mmHg with PEEP ≧ 5 cmH2O)  Exclusion Criteria:  Lack of informed consent Continuous neuromuscular blockade at enrollment DNR (do-not-resuscitate) Moribund patient not expected to survive 24 hours Massive hemoptysis Increased intracranial pressure (> 18 mmHg) Existence or high risk of pneumothorax Known pregnancy Actual body weight exceeding 1 kg/cm Patient judged to be inappropriate for the trial by intensivist