WE CLAIM:
1. An electric motor, comprising:
a rotation shaft (21),
a rotor (24) mounted to the rotation shaft,
a stator (11) mounted so as to oppose an outer peripheral surface of the rotor,
a cylindrical holder (29) mounted to an end of the rotation shaft,
a sensor magnet (28) mounted to the holder, and
a rotation sensor (16) mounted at a position at which the rotation sensor opposes the sensor magnet in a direction of an axis of rotation and detecting a rotating field of the sensor magnet;
the end of the rotation shaft where the holder is mounted has a cylindrical shape with a non-circular cross section formed of at least one plane and a curved surface in an outer peripheral portion;
at least one of an inner peripheral portion and an outer peripheral portion of the holder forms a non-circular cross section formed of a plane parallel to the axis of rotation and a curved surface connected to the plane; and
the plane of the rotation shaft and the plane of the holder are parallel to each other and the curved surface of the rotation shaft and the curved surface of the holder are in contact with each other,
characterized in that:
the sensor magnet is integrally molded with the holder at one end and fixedly supported at the one end, and
the holder is provided with at least one hole (40) in a portion where the sensor magnet is integrally molded.
2. The electric motor as claimed in claim 1, wherein:
the sensor magnet and the rotation shaft have a gap (g) in a direction of the axis of rotation via the holder.

3. The electric motor as claimed in claim 1 or 2, wherein: the holder is made of a non-magnetic material.
4. The electric motor as claimed in any one of claims 1 to 3, wherein:
the sensor magnet is shaped substantially like a cylindrical pillar with planes at both ends.
5. The electric motor as claimed in any one of claims 1 to 4, wherein:
the sensor magnet is an isotropic magnet having a residual flux density of 0.4 to 0.8 [T].
6. The electric motor as claimed in any one of claims 1 to 5, wherein:
the planar portion (39d) provided to the outer peripheral portion (41) of the holder and a plane (37b) of a fit portion (36b) for the rotation shaft are parallel each other.
7. The electric motor as claimed in any one of claims 1 to 6, wherein:
the holder has a protrusion (44), which is a part of the holder protruding in the direction of the axis of rotation from an end face of the sensor magnet on a side opposing the rotation sensor.
8. An electric power steering device, comprising the electric motor as claimed
in any one of claims 1 to 7.