To provide an intersection height measuring device of a roof mirror capable of accurately measuring the intersection height of roof mirror, without depending on the measurement accuracy of a dimension-measuring part.

The measuring device comprises an intersection height measuring part 100a of the roof mirror, provided with a flood light 17 casting light to the roof mirror and a digital dimension measuring part 19 constituted of a light receiver 18 for receiving the reflected light, a roof angle measuring part 100b of the roof mirror provided with an angle measuring part 29 for measuring the angle of the roof mirror, and a detachable reference roof mirror 26 for measuring the intersection height as a reference, of which the intersection height and roof angle are known and the roof angle as the reference. By a calculation which uses the intersection height as the reference and the roof angle as the reference, the intersection height of the roof mirror is obtained.