A rotation angle detection device has a first yoke, an inside surface of which is formed of a first concave curved surface and a second concave curved surface, and a second yoke, an inside surface of which is formed of a third concave curved surface and a fourth concave curved surface. Each concave curved surface does not extend in a second direction but extends toward a first flat surface or a second flat surface in an inclined manner. The first flat surface and the second flat surface face each other sandwiching a Hall element therebetween and are parallel to each other. The magnetic flux, which leaks from the first yoke into an inside space and reaches the second yoke, flows in the second direction over a wider area around the Hall element.