A direct-current to three-phase alternating-current inverter system comprising: a three-phase motor having three phases including a U-phase, a V-phase and a W-phase; an inverter circuit for the three-phase motor; a plurality of switching elements ( 10, 11, 12, 13, 14, 15) arranged in the inverter circuit, the switching elements ( 10, 11, 12, 13, 14, 15) respectively serving as upper arms and lower arms for the respective three phases of the three-phase motor; a capacitor connected in parallel to the respective pairs of the upper arm and lower arm; a direct-current power source arranged between the neutral-point of the three-phase motor and the respective lower arms or the respective upper arms; and a control unit controlling switching operation of the switching elements ( 10, 11, 12, 13, 14, 15), characterized in that the control circuit controls switching operation of the switching elements ( 10, 11, 12, 13, 14, 15) such that a switching frequency of each switching element ( 10, 11, 12, 13, 14, 15) is changeable to a first frequency and a second frequency which is lower than the first frequency, in a region where current flowing through the U-phase has the greatest negative value among the currents flowing through the U-phase, the V-phase and the W-phase of the three-phase motor, the switching frequency of the switching element in U-phase is changed to the second frequency and the switching frequencies of the switching elements in V-phase and W-phase are changed to the first frequency, in a region where current flowing through the V-phase has the greatest negative value among the currents flowing through the U-phase, the V-phase and the W-phase of the three-phase motor, the switching frequency of the switching element in V-phase is changed to the second frequency and the switching frequencies of the switching elements in the W-phase and U-phase are changed to the first frequency, and in region where current flowing through the W-phase has the greatest negative value among the currents flowing through the U-phase, the V-phase and the W-phase of the three-phase motor, the switching frequency of the switching element in W-phase is changed to the second frequency and the switching frequencies of the switching elements in U-phase and V-phase are changed to the first frequency, wherein the switching operation, controlled by the control circuit, serves to convert direct-current power from the capacitor into the three-phase power and supplying the three-phase alternating-current power to the three-phase motor while increasing the voltage of the direct-current power source to charge the capacitor.. a three-phase motor having three phases including a U-phase, a V-phase and a W-phase; an inverter circuit for the three-phase motor; a plurality of switching elements ( 10, 11, 12, 13, 14, 15) arranged in the inverter circuit, the switching elements ( 10, 11, 12, 13, 14, 15) respectively serving as upper arms and lower arms for the respective three phases of the three-phase motor; a capacitor connected in parallel to the respective pairs of the upper arm and lower arm; a direct-current power source arranged between the neutral-point of the three-phase motor and the respective lower arms or the respective upper arms; and a control unit controlling switching operation of the switching elements ( 10, 11, 12, 13, 14, 15), characterized in that the control circuit controls switching operation of the switching elements ( 10, 11, 12, 13, 14, 15) such that a switching frequency of each switching element ( 10, 11, 12, 13, 14, 15) is changeable to a first frequency and a second frequency which is lower than the first frequency, in a region where current flowing through the U-phase has the greatest negative value among the currents flowing through the U-phase, the V-phase and the W-phase of the three-phase motor, the switching frequency of the switching element in U-phase is changed to the second frequency and the switching frequencies of the switching elements in V-phase and W-phase are changed to the first frequency, in a region where current flowing through the V-phase has the greatest negative value among the currents flowing through the U-phase, the V-phase and the W-phase of the three-phase motor, the switching frequency of the switching element in V-phase is changed to the second frequency and the switching frequencies of the switching elements in the W-phase and U-phase are changed to the first frequency, and in region where current flowing through the W-phase has the greatest negative value among the currents flowing through the U-phase, the V-phase and the W-phase of the three-phase motor, the switching frequency of the switching element in W-phase is changed to the second frequency and the switching frequencies of the switching elements in U-phase and V-phase are changed to the first frequency, wherein the switching operation, controlled by the control circuit, serves to convert direct-current power from the capacitor into the three-phase power and supplying the three-phase alternating-current power to the three-phase motor while increasing the voltage of the direct-current power source to charge the capacitor..