Rotation angle sensor

1. A rotation angle sensor comprising:

a magnet that rotates about a rotation axis, the magnet having an end face perpendicular to the rotation axis and having a magnetization in a direction perpendicular to the rotation axis; and

a magnetic sensor that faces the end face of the magnet and detects a magnetic field produced by the magnet,

the rotation angle sensor detecting a rotation angle of the magnet based on a detection output from the magnetic sensor, wherein:

the magnet includes:

a plate-shaped portion including the end face of the magnet; and

a ring-shaped portion that is located on a side of the plate-shaped portion farther from the end face and coupled to the plate-shaped portion, and

the plate-shaped portion does not include any hollow through which the rotation axis passes, whereas the ring-shaped portion includes a hollow through which the rotation axis passes, the rotation angle sensor being configured so that on a virtual plane that is parallel to the end face of the magnet and apart from the end face by a distance equal to a distance between the end face and the magnetic sensor, the magnetic field produced by the magnet is in directions parallel to each other at a first position and a second position, the first position being a position at which the rotation axis intersects the virtual plane, the second position being different from the first position.

2. The rotation angle sensor according to claim 1 , wherein any cross section of the magnet including the rotation axis has a shape symmetric about the rotation axis.

3. The rotation angle sensor according to claim 2 , wherein the plate-shaped portion is shaped like a circular plate, and the ring-shaped portion has an outer periphery and an inner periphery that are both circular in any cross section of the ring-shaped portion perpendicular to the rotation axis.

4. The rotation angle sensor according to claim 1 , wherein the magnetic sensor includes a magnetoresistive element.