A control device for an alternating-current motor, in which an applied voltage is controlled by an inverter, the control device comprising: a pulse width modulation control unit for generating a control command (S3-S8) for said inverter by performing pulse width modulation control based on a comparison between a voltage command signal of sinusoidal wave for operating said alternating-current motor in accordance with an operation command and a carrier wave signal, and a mode-switching determining unit for indicating which one of an overmodulation mode and a sinusoidal wave modulation mode is used for said pulse width modulation control performed by said pulse width modulation control unit, in said overmodulation mode, said voltage command signal having an amplitude larger than an amplitude of said carrier wave signal and a frequency of said carrier wave signal being controlled in accordance with rotation speed of said alternating-current motor such that the frequency of said carrier wave signal is an integral multiple of a rotational frequency of said alternating-current motor, in said sinusoidal wave modulation mode, said voltage command signal having an amplitude equal to or smaller than the amplitude of said carrier wave signal and the frequency of said carrier wave signal being set irrespective of the rotation speed of said alternating-current motor, and the mode-switching determining unit being configured to determine whether to switch from said overmodulation mode to said sinusoidal wave modulation mode, based on a comparison between a value concerned with said voltage command signal and a switching determination value, wherein the value concerned with said voltage command signal and said switching determination value are indicated by: (i) a modulation factor determined by a direct-current voltage input to said inverter and a voltage command value (Vdcom, Vqcom) based on which said voltage command signal is generated; (ii) a parameter associated with the voltage command value; or (iii) an amplitude or phase of a voltage applied to the alternating current motor; and the control device being characterized in that it further comprises a determination value setting unit for variably setting the switching determination value (Fjd) for determining switching from said overmodulation mode to said sinusoidal wave modulation mode, based on a state of a power conversion operation performed by said inverter, when said pulse width modulation control is performed in accordance with said overmodulation mode a pulse width modulation control unit for generating a control command (S3-S8) for said inverter by performing pulse width modulation control based on a comparison between a voltage command signal of sinusoidal wave for operating said alternating-current motor in accordance with an operation command and a carrier wave signal, and a mode-switching determining unit for indicating which one of an overmodulation mode and a sinusoidal wave modulation mode is used for said pulse width modulation control performed by said pulse width modulation control unit, in said overmodulation mode, said voltage command signal having an amplitude larger than an amplitude of said carrier wave signal and a frequency of said carrier wave signal being controlled in accordance with rotation speed of said alternating-current motor such that the frequency of said carrier wave signal is an integral multiple of a rotational frequency of said alternating-current motor, in said sinusoidal wave modulation mode, said voltage command signal having an amplitude equal to or smaller than the amplitude of said carrier wave signal and the frequency of said carrier wave signal being set irrespective of the rotation speed of said alternating-current motor, and the mode-switching determining unit being configured to determine whether to switch from said overmodulation mode to said sinusoidal wave modulation mode, based on a comparison between a value concerned with said voltage command signal and a switching determination value, wherein the value concerned with said voltage command signal and said switching determination value are indicated by: (i) a modulation factor determined by a direct-current voltage input to said inverter and a voltage command value (Vdcom, Vqcom) based on which said voltage command signal is generated; (ii) a parameter associated with the voltage command value; or (iii) an amplitude or phase of a voltage applied to the alternating current motor; and the control device being characterized in that it further comprises (i) a modulation factor determined by a direct-current voltage input to said inverter and a voltage command value (Vdcom, Vqcom) based on which said voltage command signal is generated; (ii) a parameter associated with the voltage command value; or (iii) an amplitude or phase of a voltage applied to the alternating current motor; and the control device being characterized in that it further comprises a determination value setting unit for variably setting the switching determination value (Fjd) for determining switching from said overmodulation mode to said sinusoidal wave modulation mode, based on a state of a power conversion operation performed by said inverter, when said pulse width modulation control is performed in accordance with said overmodulation mode