An inverter-controlling device for controlling an inverter system, the inverter system comprising a rectifying module for rectifying power supplied from an alternate current power supply, a smoothing module for smoothing an output voltage from the rectifying module and an inverting module including a plurality of switching elements for converting the smoothened voltage from the smoothing module into an AC voltage and for supplying the AC voltage to a motor, wherein the inverter-controlling device comprises: a main controller configured to, when the supply of the AC voltage to the motor stops, generate a command current of a first predetermined magnitude; a comparison module configured to calculate a difference between the command current and an output current of the inverting module; a current controller configured to perform a proportional-integral control based on the difference; and a frequency estimation module configured to, while the command current is injected to the current controller, estimate a rotation speed of the motor based on current responses to the command currents, characterized in that the current responses to the command currents include an A-phase current response, a B-phase current response and a C-phase current response, wherein a magnitude of the A-phase current response corresponds to twice a magnitude of each of the B-phase current response and of the C-phase current response, wherein the A-phase current response is a positive ripple current, while each of the B-phase current response and the C-phase current response is a negative ripple current, the B-phase current response and the C-phase current response are out-of-phases in an opposite direction, wherein the main controller is further configured to: when a flying start operation of the inverting module begins, set an output frequency of the inverting module to a first frequency greater by at least a second predetermined magnitude than the rotation speed of the motor estimated by the frequency estimation module; decrease the output frequency of the inverting module from the first frequency to the estimated rotation speed of the motor; and increase an output voltage of the inverting module from zero. a main controller configured to, when the supply of the AC voltage to the motor stops, generate a command current of a first predetermined magnitude; a comparison module configured to calculate a difference between the command current and an output current of the inverting module; a current controller configured to perform a proportional-integral control based on the difference; and a frequency estimation module configured to, while the command current is injected to the current controller, estimate a rotation speed of the motor based on current responses to the command currents, characterized in that the current responses to the command currents include an A-phase current response, a B-phase current response and a C-phase current response, wherein a magnitude of the A-phase current response corresponds to twice a magnitude of each of the B-phase current response and of the C-phase current response, wherein the A-phase current response is a positive ripple current, while each of the B-phase current response and the C-phase current response is a negative ripple current, the B-phase current response and the C-phase current response are out-of-phases in an opposite direction, wherein the main controller is further configured to: when a flying start operation of the inverting module begins, set an output frequency of the inverting module to a first frequency greater by at least a second predetermined magnitude than the rotation speed of the motor estimated by the frequency estimation module; decrease the output frequency of the inverting module from the first frequency to the estimated rotation speed of the motor; and increase an output voltage of the inverting module from zero. when a flying start operation of the inverting module begins, set an output frequency of the inverting module to a first frequency greater by at least a second predetermined magnitude than the rotation speed of the motor estimated by the frequency estimation module; decrease the output frequency of the inverting module from the first frequency to the estimated rotation speed of the motor; and increase an output voltage of the inverting module from zero.