An angle detection device includes a rotator including a shaft supported by and inside a case in a direction parallel to a horizontal plane of the case, a center of gravity positioned differently from the shaft, and a diagonal line formed on a surface along a circumferential direction around a rotation axis of the rotator, the line intersecting with a width direction orthogonal to the circumferential direction if the surface is spread into a plane along the circumferential direction; and a sensor including a group of imaging elements arrayed in a line parallel to the shaft direction. The sensor is fixed to and inside the case opposite to the surface. The group intersects with the diagonal line if the surface is viewed from the sensor toward the shaft, and a position of the intersection changes according to rotation of the case around the axis when viewed from the shaft.