A detecting device for a fuel injector includes a sensor unit and an ECU. The sensor unit is provided with a fuel pressure detection circuit which outputs a pressure detection signal in response to a fuel pressure. The ECU computes the fuel pressure based on a voltage value of the pressure detection signal relative to a reference voltage. The ECU obtains a comparative voltage according to an applied-voltage to the fuel pressure detection circuit and computes a deviation between the comparative voltage and the reference voltage. The sensor unit adjusts the applied-voltage in such a manner as to decrease the deviation. Thus, the computation accuracy of the fuel pressure is improved.