Abstract: To provide a rotation detection device that has a miniaturized sensor IC in comparison with a prior art. A rotation detection device includes: a magnet that forms a magnetic field toward a tooth surface of a gear; and a sensor disposed between the magnet and the gear, the sensor including: at least a pair of magnetic detection elements x1 and x2 that outputs signals according to a magnetic flux density in a radial direction of the gear; and a magnetic concentrator that induces, in the radial direction of the gear, a component in a circumferential direction of the gear in the magnetic flux density on detection surfaces of the magnetic detection elements x1 and x2, the sensor detecting a variation in a magnetic flux density accompanying the rotation of the gear.