A non-invasive manometer comprises a bladder to be attached to an appendage such as a finger, a leg, a tail or the like in order to apply a pressure thereto. A pressure changing device changes the pressure. Two pairs of photoelectric sensors sense a change in the diameter of the blood vessel due to the applied pressure and the internal pressure of the blood vessel. Each sensor includes a light emitting unit and a photosensitive unit. At least one of the units includes a plurality of elements. The photoelectric sensors are shielded from possible external light and operated alternately in a time-divisional manner. One sensor is disposed closer to the heart than the other sensor. The output of the other sensor is used to determine the highest blood pressure while the outputs of both the sensors are compared to determine the lowest blood pressure.