PURPOSE: To lower the cost of a detector and to enhance the reliability thereof by adding the voltage induced in the secondary side output part of a differential transformer by the magnetic flux of a primary coil in the same or reverse phase with respect to the output of a secondary coil.

CONSTITUTION: A coil 10 adding the voltage induced in the secondary side output part of a differential transformer 2 by the magnetic flux of a primary coil 3 to the outputs of secondary coils 4, 5 in the same phase or reverse phase is provided in series to the transformer 2. When acceleration/deceleration speed x is 0, the synthetic output voltage V on the secondary side is composed only of the voltage generated in the series coil 10 and V=V0 sin 2π ft(f;power supply frequency) is formed. The synthetic output voltage when a movable magnetic body 6 receives acceleration to be displaced becomes V=(V0+Kx) sin 2πft(K; proportion constant). When the magnetic body 6 receives deceleration to be displaced, the synthetic output voltage becomes V=(V0-Kx)sin2πft. By passing the synthetic output voltage thus obtained through an amplifying circuit 11, a rectifying circuit 12 and a filter circuit 13, output voltage is obtained as V=V0+Kx(V0-Kx at the time of deceleration) in a final output means.