PURPOSE: To prevent a motor for an electric compressor from stopping and greatly changing its number of revolutions due to an overload and a magnetic overexcitation thereof even if a power supply voltage is greatly changed according to the state of the running of an automobile by cheaply insulating a high voltage system from a low voltage system, keeping a small size, a light weight and spacing saving, decreasing a manufacture cost; and simplifying administration of parts.

CONSTITUTION: A D.C. voltage detecting means 10 is provided, by comprising D.C. voltage generating means 1 for generating a D.C. voltage, an electric compressor 9 endosing a motor driven by electrical input a D.C./A.C. conversion means 8 for converting the D.C. voltage to a simulated A.C. voltage in its state of plus and minus pulse line, a window.comparator circuit 4 for making a first voltage obtained by dividing the DC voltage by a resistance as one side input, and making a standard voltage as other input; and insulating means 5 for insulating an output of the window.comparator circuit 4 from the D.C. voltage generating means 1.