To obtain response with high sensitivity without being affected by an external magnetic field.

The steel sphere detector comprises a first yoke 2 having a first surrounding part 2a for partly surrounding a passing steel sphere 10 and a first magnetic path 2b extended from the part 2a, and a second yoke 3 having a second surrounding part 3a for surrounding the sphere 10 together with the part 2a provided separately from the yoke 2 and a second magnetic path 3b extended from the part 3a and disposed oppositely to the path 2b. The detector further comprises a permanent magnet 4 sandwiched between the paths 2b and 3b near the parts 2a and 3a, and a Hall IC 5 disposed separately from the parts 2a and 3a from the magnet 5. Thus, when the magnetic flux density detected by the IC 5 is reduced, the passage of the sphere can be detected and hence it is not affected by the external magnetic field.