Rotation angle detecting device and electric actuator

A rotation angle detecting device, capable of detecting a rotation angle of a rotation body rotatable about a central axis, the rotation angle detecting device comprising:

• a sensor magnet, having a plurality of magnetic poles provided side-by side in a circumferential direction about the central axis and installed to the rotation body;
• a magnetic sensor, overlapped with the sensor magnet when viewed in a predetermined first direction and capable of detecting a magnetic field of the sensor magnet;
• and
• a pair of magnetic parts, provided to sandwich the magnetic sensor in a second direction intersecting the first direction,
• wherein the first direction and the second direction are directions orthogonal to each other,
• wherein the first direction is an axial direction of the central axis, and the second direction is the circumferential direction about the central axis.