A non-invasive monitor for measuring regional saturation of oxygen includes a sensor unit containing a printed circuit board on which a light emitting unit and a light receiving unit are mounted； a main body unit； a sensor holder for holding the sensor unit while the light emitting unit and the light receiving unit are disposed in an aperture portion； a sensor pressing board； a connecting unit for electrically connecting the sensor unit and the main body unit； and a headband. The light emitting unit and the light receiving unit are disposed such that a light emitting surface and a light receiving surface face the forehead-side, and a part or the whole of the forehead-side surface of the sensor unit is on the same surface as the forehead-side surface of the sensor holder or protrudes from the forehead-side surface of the sensor holder toward the forehead-side.
