The present invention has a normal blood pressure measurement mode in which a blood pressure measurement is performed according to a blood pressure measurement instruction, and a nighttime blood pressure measurement mode in which a blood pressure measurement is automatically started according to a predetermined schedule. When being switched by an mode operation unit from the normal blood pressure measurement mode to the nighttime blood pressure measurement mode, an algorithm for blood pressure calculation by an oscillometric method is switched and set from an algorithm for a sitting position to an algorithm for a supine position. A blood pressure value is calculated using an algorithm currently set for the blood pressure calculation by the oscillometric method.