A method of controlling a motor (9, 9A) driven by a three-phase voltage source inverter (2, 2A), the method comprising the steps of: a) acquiring a rotational speed parameter representing a rotational speed of the motor (9, 9A); b) after step a), selecting one of a first system and a second system as a switching signal generation system based on a result of comparing the rotational speed of the motor (9, 9A) represented by the rotational speed parameter with a predetermined reference speed; c) after step a), calculating phase voltage command values (S49, S49A) for each of three phases of the motor (9, 9A) based on a target rotational speed (S40, S40A) and the rotational speed parameter; d) after steps b) and c), generating switching signals (S4, S4A) for each of the three phases of the motor (9, 9A) by employing the selected switching signal generation system based on the phase voltage command values (S49, S49A); and e) after step d), outputting the switching signals (S4, S4A) to the inverter (2, 2A); wherein the first system is a three-phase modulation scheme which calculates duties of the switching signals (S4, S4A) for each of the three phases of the motor (9, 9A) based on voltage values of the phase voltage command values (S49, S49A) for each of the three phases of the motor (9, 9A) and a voltage value of a carrier signal; wherein the second system is a two-phase modulation scheme which, based on the voltage values of the phase voltage command values (S49, S49A) for each of the three phases and the voltage value of the carrier signal, selects one of the three phases and fixes the duties of the switching signals (S4, S4A) for the selected phase in an ON state or an OFF state, and calculates the duties of the switching signals (S4, S4A) for the other two phases; wherein in step b), the second system is selected when the rotational speed is higher than the reference speed; characterized in that step a) includes: a-1) detecting a shunt current (Is) which flows through a shunt resistor (Rs) provided in the inverter; a-2) calculating estimated three-phase currents based on the shunt current (Is); a-3) transforming the estimated three-phase currents to an αβ stationary reference frame to calculate stationary reference frame currents; and a-4) transforming the stationary reference frame currents to a dq synchronously rotating reference frame to calculate rotating reference frame currents as the rotational speed parameter; the shunt resistor (Rs) is a resistor connected in series with a ground line common to all the three phases of the inverter; the method further comprises a step of f) after step a) and before step d), comparing the rotational speed of the motor with a predetermined threshold speed based on the rotational speed parameter, and determining whether or not a current correction is to be performed in step d) based on a result of this comparison; the shunt current (Is) is measured at two points in time during one control cycle to estimate the phase currents of the three phases; and the current correction is an adjustment of ON/OFF switching timing of one or two pairs of switching signals among the three pairs of switching signals included in the switching signals. a) acquiring a rotational speed parameter representing a rotational speed of the motor (9, 9A); b) after step a), selecting one of a first system and a second system as a switching signal generation system based on a result of comparing the rotational speed of the motor (9, 9A) represented by the rotational speed parameter with a predetermined reference speed; c) after step a), calculating phase voltage command values (S49, S49A) for each of three phases of the motor (9, 9A) based on a target rotational speed (S40, S40A) and the rotational speed parameter; d) after steps b) and c), generating switching signals (S4, S4A) for each of the three phases of the motor (9, 9A) by employing the selected switching signal generation system based on the phase voltage command values (S49, S49A); and e) after step d), outputting the switching signals (S4, S4A) to the inverter (2, 2A); a-1) detecting a shunt current (Is) which flows through a shunt resistor (Rs) provided in the inverter; a-2) calculating estimated three-phase currents based on the shunt current (Is); a-3) transforming the estimated three-phase currents to an αβ stationary reference frame to calculate stationary reference frame currents; and a-4) transforming the stationary reference frame currents to a dq synchronously rotating reference frame to calculate rotating reference frame currents as the rotational speed parameter;