Abstract: In an electronic control type automatic speed changer equipped with a linear solenoid for controlling the oil pressure of the automatic speed changer, when potential difference is generated between the power ground (PE) and the signal ground (SE), the line pressure of the automatic speed changer does not coincide with the predetermined set value due to erroneous signals generated. In order to compensate for the potential difference between PE and SE, according to the present invention, a PE -SE potential difference compensator is provided in series between a current monitor device and PE, and a solenoid monitor circuit is provided for monitoring the potential of the connection point of the current monitor device and the PE-SE potential difference compensator. Thus even if there is a potential difference between PE and SE, a potential difference proportional to the current actually flowing in the linear solenoid can be detected.