PURPOSE: To facilitate adjustment by constituting a bridge circuit by two detector coils together with two balancing resistances and producing an output signal when unbalance is caused in the bridge circuit due to the displacement of the control sleeve.

CONSTITUTION: Detecting coils L1, L2 are wound around a U-shaped iron core 3 which is disposed adjacently of a control sleeve 2 fitted on the plunger 1 of a fuel injection pump. A bridge circuit is formed by the detecting coils L1, L2 together with balancing resistances Ra, Rb so that an output signal is produced from an oscillation circuit A and a voltage amplifying circuit B when unblance is caused in the bridge circuit L1, L2, Ra, Rb. This output signal is converted into an output voltage proportional to the displacement of control sleeve 2 by means of an absolute- value circuit C and a smoothing circuit D. With such an arrangement, it is enabled to eliminate influences of the temperature and to use a common control sleeve 2.