Abstract: A biological data observation system makes it possible to accurately observing the anesthetic depth of a patient. The system measures a signal based on the pulse wave of the patient, and measures the sweat rate. During a surgery, these data and/or their composite data is monitored. The system displays transition of the sweat rate. Therefore, if during the surgery the patient's condition changes, an operating doctor (operator) will immediately notice a change in the sweat rate displayed by the system, so that the doctor or the like can promptly take an appropriate measure for the change in the patient's condition. By observing the transition of the patient's blood pressure displayed by the system, the doctor can also understand how the patient's condition changes as a result of the doctor's measure.