To provide a magnetic field detecting device using a plurality of magnetic field detecting parts with high heat resistance, simple structure, and small dimension, capable of processing signals from the magnetic field detecting parts with one magnetic field decision part at the same time, and knowing the number of the defective parts of the magnetic field.

This magnetic field detecting device includes a magnetic field detecting part group, a power source supplying AC to the magnetic field detecting part group, and the magnetic field decision part deciding a state of magnetic field acting to the magnetic field detecting part group. The magnetic field detecting part group includes a plurality of magnetic field detecting parts each having a magnetic core and a coil arranged in the outer circumference of the magnetic core, and connects the coils in series and receives AC from the power source to the coil. The magnetic field decision part detects current flowing to the magnetic field detecting part group or voltage across the magnetic field detecting part group, decides the magnitude relation of a detected signal and at least one reference value previously set, and outputs a decision signal corresponding to a decided result.