To provide a corrected resistance reading method capable of simplifying a constitution for reading a resistance value for a corrected resistance provided for correcting the fuel injection characteristics of an injector in a signal generating means preparing an injection signal to the injector.

A diag-tool is equipped with a communication line 1b for letting the register value of a corrected resistance 54 for respective injectors 5, and with a communication line 1a to be communicated with an ECU 3. This diag- tool can read in each register value from the respective injectors 5 when a power supply switch is turned on by way of the communication line 1b, and can transmit messages with the ECU 3 by way of the communication line 1a. The use of the diag-tool 1 thereby enables each register valve to be read in the ECU 3 without providing any communication line in continuous length only for reading in each register value, between the ECU 3 and the injector 5.