A thyristor starter for starting a synchronous motor comprising: a converter configured to convert a first three-phase AC power to DC power; a DC reactor configured to smooth said DC power; an inverter configured to convert said DC power supplied from said converter through the intermediary of said DC reactor to a second three-phase AC power, and supply said second three-phase AC power to said synchronous motor through first to third AC lines (RL, SL, TL); a ground fault detecting circuit configured to detect a ground fault in said thyristor starter; and a control circuit configured to control said converter and said inverter to start said synchronous motor, and to stop the operation of said converter and said inverter when said ground fault has been detected by said ground fault detecting circuit, said ground fault detecting circuit including first to fourth resistance elements (31 to 34), one terminals of said first to third resistance elements (31 to 33) being connected to said first to third AC lines (RL, SL, TL), respectively, the other terminals of three of said resistance elements (31 to 33) being commonly connected to one terminal of said fourth resistance elements, and the other terminal of said fourth resistance element being configured to receive a ground voltage, said converter including first and second terminals (3a, 3b) configured to output DC voltage, said inverter including third and fourth terminals (5a, 5b) configured to receive the DC voltage, said DC reactor being connected between said first and third terminals (3a, 5a), said second and fourth terminals (3b, 5b) being connected to each other, a voltage across the terminals of said fourth resistance element being about 0 V when no ground fault occurs, and the voltage across the terminals of the fourth resistance element increasing in amplitude when the ground fault occurs at at least one of said first to third terminals (3a, 3b, 5a) and said first to third AC lines (RL, SL, TL), and said ground fault detecting circuit further including a determination circuit configured to determine that said ground fault has occurred at at least one of said first to third terminals (3a, 3b, 5a) and said first to third AC lines (RL, SL, TL) when a voltage across the terminals of said fourth resistance element is greater than a predetermined voltage. a converter configured to convert a first three-phase AC power to DC power; a DC reactor configured to smooth said DC power; an inverter configured to convert said DC power supplied from said converter through the intermediary of said DC reactor to a second three-phase AC power, and supply said second three-phase AC power to said synchronous motor through first to third AC lines (RL, SL, TL); a ground fault detecting circuit configured to detect a ground fault in said thyristor starter; and a control circuit configured to control said converter and said inverter to start said synchronous motor, and to stop the operation of said converter and said inverter when said ground fault has been detected by said ground fault detecting circuit, said ground fault detecting circuit including first to fourth resistance elements (31 to 34), one terminals of said first to third resistance elements (31 to 33) being connected to said first to third AC lines (RL, SL, TL), respectively, the other terminals of three of said resistance elements (31 to 33) being commonly connected to one terminal of said fourth resistance elements, and the other terminal of said fourth resistance element being configured to receive a ground voltage, said converter including first and second terminals (3a, 3b) configured to output DC voltage, said inverter including third and fourth terminals (5a, 5b) configured to receive the DC voltage, said DC reactor being connected between said first and third terminals (3a, 5a), said second and fourth terminals (3b, 5b) being connected to each other, a voltage across the terminals of said fourth resistance element being about 0 V when no ground fault occurs, and the voltage across the terminals of the fourth resistance element increasing in amplitude when the ground fault occurs at at least one of said first to third terminals (3a, 3b, 5a) and said first to third AC lines (RL, SL, TL), and said ground fault detecting circuit further including a determination circuit configured to determine that said ground fault has occurred at at least one of said first to third terminals (3a, 3b, 5a) and said first to third AC lines (RL, SL, TL) when a voltage across the terminals of said fourth resistance element is greater than a predetermined voltage.