A sensor unit includes a support body having a concave surface, a spherical body formed of a magnetic material and placed on the concave surface of the support body so as to roll freely thereon, a permanent magnet producing a magnetic field affecting the magnetic spherical body, and a magnetic sensor detecting a change in the magnetic field caused by the movement of said magnetic spherical body. The magnetic spherical body and the permanent magnet are provided to oppose each other with a given distance therebetween in a vertical direction. The magnetic sensor is provided between the magnetic spherical body and the permanent magnet. An output is produced in accordance with the detection by the magnetic sensor.