Abstract: An inclination angle detector electrically detects an angle of inclination of an object of measurement with respect to the horizontal direction or the direction of gravitation. An inclination angle detector includes a bubble tube having a transparent container formed such that the inner surface of an upper part thereof has a spherical concave surface. The transparent container has a gas and a transparent liquid sealed therein. A light-emitting unit emits light towards the bubble tube from the upper or lower side thereof. A light-receiving element converts the light transmitted by the bubble tube into an electric signal. An operating unit calculates an angle of inclination based on the electric signal. The light-receiving element includes a light-receiving unit for position detection within a movable range of a bubble shadow and a light-receiving unit for reference outside the movable range.