An inverter apparatus , which is to be connected to an armature winding of an AC electric motor , and is configured to control the AC electric motor , wherein the inverter apparatus comprising a winding switching signal generator configured to output a winding switching signal for interchangeably switching a state of the armature winding between a first winding and a second winding, to a winding switch configured to switch connection of the armature winding so that the state of the armature winding is interchangeably switched between the first winding and the second winding, wherein the first winding is chosen when a rotation speed (ω) of a rotor of the AC electric motor is in a low speed region, and the second winding is chosen when the rotation speed (ω) is in a high speed region, the winding switching signal generator is configured to output the winding switching signal at a first switch timing when the rotation speed (ω) becomes lower than a first rotation speed (ωa) in a case where a current state of the armature winding is the second winding, and to output the winding switching signal at a second switch timing when the rotation speed (w) becomes higher than a second rotation speed (ωb) that is higher than the first rotation speed (ωa) in a case where the current state of the armature winding is the first winding, the winding switching signal generator being characterised in that it comprises a hysteresis region judging section configured to judge whether or not a current rotation speed (w) is within a hysteresis region which is a speed region between the first rotation speed (ωa) and the second rotation speed (ωb); a winding comparator configured to judge, when the current rotation speed (ω) is within the hysteresis region, whether or not a current winding which is the current state of the armature winding that has been chosen from among the first winding and the second winding is the same as an optimal winding which is one of the first winding and the second winding with which an energy efficiency of the AC electric motor is higher than that of the other, the energy efficiency being determined based on the current winding and the current rotation speed (ω); and a winding switching signal output section configured to output, when the current rotation speed (w) is within the hysteresis region and when the current winding differs from the optimal winding, the winding switching signal so that the current winding is switched to the optimal winding. the winding switching signal generator being characterised in that it comprises a hysteresis region judging section configured to judge whether or not a current rotation speed (w) is within a hysteresis region which is a speed region between the first rotation speed (ωa) and the second rotation speed (ωb); a winding comparator configured to judge, when the current rotation speed (ω) is within the hysteresis region, whether or not a current winding which is the current state of the armature winding that has been chosen from among the first winding and the second winding is the same as an optimal winding which is one of the first winding and the second winding with which an energy efficiency of the AC electric motor is higher than that of the other, the energy efficiency being determined based on the current winding and the current rotation speed (ω); and a winding switching signal output section configured to output, when the current rotation speed (w) is within the hysteresis region and when the current winding differs from the optimal winding, the winding switching signal so that the current winding is switched to the optimal winding. <EOS>