An electric motor system, comprising: a synchronous electric motor ; and a power conversion device configured to drive the synchronous electric motor ; wherein the power conversion device is configured to determine whether the synchronous electric motor is out of synchronization or not based on a change amount of a DC voltage value or an output current value inside the power conversion device when a rotational speed of the synchronous electric motor is changed by a predetermined amount to return to an original value during operation of the synchronous electric motor , characterized in that a condition of permission for restart of the synchronous electric motor comprises a number (CN) and occurrence frequency of detected synchronization losses, wherein the power conversion device is configured to process the following conditions: when a frequency is added to the condition of permission for restart, a set value of a timer (TM2) for confirmation of a synchronization loss frequency, stored in advance at an address in a storage unit of the power conversion device at a time of detecting a loss of synchronization, is stored in a remaining time (TN2) of the timer (TM2) at an address in the storage unit , and the remaining time (TN2) is counted down, when a loss of synchronization is again detected before the remaining time (TN2) becomes 0, a condition that restart is not permitted is added. a synchronous electric motor ; and a power conversion device configured to drive the synchronous electric motor ; wherein the power conversion device is configured to determine whether the synchronous electric motor is out of synchronization or not based on a change amount of a DC voltage value or an output current value inside the power conversion device when a rotational speed of the synchronous electric motor is changed by a predetermined amount to return to an original value during operation of the synchronous electric motor , characterized in that a condition of permission for restart of the synchronous electric motor comprises a number (CN) and occurrence frequency of detected synchronization losses, wherein the power conversion device is configured to process the following conditions: when a frequency is added to the condition of permission for restart, a set value of a timer (TM2) for confirmation of a synchronization loss frequency, stored in advance at an address in a storage unit of the power conversion device at a time of detecting a loss of synchronization, is stored in a remaining time (TN2) of the timer (TM2) at an address in the storage unit , and the remaining time (TN2) is counted down, when a loss of synchronization is again detected before the remaining time (TN2) becomes 0, a condition that restart is not permitted is added. when a frequency is added to the condition of permission for restart, a set value of a timer (TM2) for confirmation of a synchronization loss frequency, stored in advance at an address in a storage unit of the power conversion device at a time of detecting a loss of synchronization, is stored in a remaining time (TN2) of the timer (TM2) at an address in the storage unit , and the remaining time (TN2) is counted down, when a loss of synchronization is again detected before the remaining time (TN2) becomes 0, a condition that restart is not permitted is added. <EOS>