Acceleration sensor and magnetic disk drive apparatus

A magnetic disk drive apparatus with an acceleration sensor comprising:

• a pair of permanent magnets each having a surface facing a magnetic field detection sensor, said pair of permanent magnets being arranged in parallel so that said surfaces of the pair of permanent magnets have different magnetic polarities with each other;
• a spring member for supporting said pair of permanent magnets to displace the pair of permanent magnets when an external force is applied;
• and
• the magnetic field detection sensor mounted in stationary state to face said pair of permanent magnets,
• said magnetic field detection sensor having at least one pair of multi-layered magnetoresistive effect elements, each including a magnetization fixed layer and a magnetization free layer, said magnetization fixed layer being magnetized in a direction parallel to a displacement detection direction,
• said pair of permanent magnets being arranged so that a longitudinal direction of each permanent magnet is in parallel with the magnetized direction of said magnetization fixed layer.