To improve the accuracy for detecting the turning of a vehicle by operating a partially integrated value and total integrated values during the period of operating an output voltage and using an average value during the period of operating differences between the total integrated values and DC elimination voltage integrated values as a static voltage when the partially integrated value approximately coincides with the total integrated values.

An output voltage Vout' during a period tst, a DC elimination voltage Vou.st' an output voltage Vout.end during a period tend and DC elimination voltages Vout.end' are respectively inputted to a processing circuit. A DC elimination voltage integrated value V' during an operation period T from the period tst to tend and total integrated values Vtotal of output voltages Vout are operated. Also, a partially integrated value of a voltage obtained by subtracting a voltage Vout.st. from a voltage Vout during the period T is operated. The integrated values V and V' are compared with each other and when these values approximate coincide with each other, an average value among differences between the integrated value Vtotal and the integrated value V' during the period T is set as a static voltage Vref.