A power converter comprising: a converter configured to convert a first AC power to a DC power; a DC reactor configured to smooth a direct current; an inverter configured to convert said DC power supplied from said converter via said DC reactor to a second AC power which is supplied to a load; a first AC voltage detector configured to detect a first AC voltage input to said converter; a second AC voltage detector configured to detect a second AC voltage output from said inverter; and a control circuit configured to control said converter and said inverter on the basis of a detection result from said first AC voltage detector and said second AC voltage detector, wherein each of said first AC voltage and said second AC voltage is a three-phase AC voltage, the frequency of said second AC voltage is variable, said load is a synchronous motor, said power converter is a thyristor starter for starting said synchronous motor, each of said converter and said inverter includes a plurality of thyristors (21-26 or 31-36), said second AC power is supplied to said synchronous motor via a first phase line, a second phase line, and a third phase line, and said control circuit is configured to generate a plurality of control signals (φ31-φ36) for controlling said plurality of thyristors included in said inverter on the basis of a detection result from said second AC voltage detector, and determine a DC voltage input to said inverter on the basis of said plurality of control signals (φ31-φ36) and the detection result from said second AC voltage detector including a first line voltage (Vrs) between the first phase line and the second phase line and a second line voltage (Vst) between the second phase line and the third phase line. a converter configured to convert a first AC power to a DC power; a DC reactor configured to smooth a direct current; an inverter configured to convert said DC power supplied from said converter via said DC reactor to a second AC power which is supplied to a load; a first AC voltage detector configured to detect a first AC voltage input to said converter; a second AC voltage detector configured to detect a second AC voltage output from said inverter; and a control circuit configured to control said converter and said inverter on the basis of a detection result from said first AC voltage detector and said second AC voltage detector, wherein each of said first AC voltage and said second AC voltage is a three-phase AC voltage, the frequency of said second AC voltage is variable, said load is a synchronous motor, said power converter is a thyristor starter for starting said synchronous motor, each of said converter and said inverter includes a plurality of thyristors (21-26 or 31-36), said second AC power is supplied to said synchronous motor via a first phase line, a second phase line, and a third phase line, and said control circuit is configured to generate a plurality of control signals (φ31-φ36) for controlling said plurality of thyristors included in said inverter on the basis of a detection result from said second AC voltage detector, and determine a DC voltage input to said inverter on the basis of said plurality of control signals (φ31-φ36) and the detection result from said second AC voltage detector including a first line voltage (Vrs) between the first phase line and the second phase line and a second line voltage (Vst) between the second phase line and the third phase line.