Magnetism detection device and absolute encoder

A magnetism detection device, comprising:

• a magnet magnetized;
• a magnetic sensor configured to detect a magnetic flux from the magnet;
• a magnet holder holding the magnet;
• a shaft, and
• a base, wherein a lower end of the shaft is press-fitted into a shaft supporting portion of the base,
• wherein;
• the magnet holder is rotatably supported on the shaft,
• the shaft is made of a magnetic material,
• an attractive force due to a magnetic force is generated between the magnet and the shaft in a direction of a rotation axis of the magnet holder,
• a snap ring is configured to restrict movement of the magnet holder in the axial direction of the shaft,
• the magnet holder, that rotates around the rotation axis, includes a gear and a magnet holder portion, wherein the magnet is clamped and fixed between the gear and the magnet holder portion,
• the shaft is inserted into the through hole of a main body portion of the gear,
• an annular groove is formed around the axial line of the shaft in a portion at the upper end side of the shaft,
• the snap ring is formed to be engageable with the annular groove to restrict movement of the magnet holder, and
• in an inverted state, the magnet holder is held with a lower end surface of the gear in contact with an upper surface of the base, so as to maintain a position of the magnet holder in a central axis direction in an upright state and prevent the magnet from moving to a magnetic sensor side.