The invention provides a synchronous electromotor driving device which smoothly starts an acceleration synchronous electromotor without speed and a position speed, and restrains reversion. The synchronous electromotor driving device of the invention controls magnitude, frequency, phase of an output voltage, addes a gived phase corrected value to the phase when a synchronous electromotor is started, modifies the phase corrected value more than once, and corrects the frequency or phase based on a detection value of an output current. In addition, when the phase corrected value is modified, the phase is increased at a positive rotation direction of the electromotor, for example, gradually incresed to lower than 90 degrees. Morever, when the gived phase corrected value initially modified, a time setting an initial phase corrected value is shorter than a time setting a modified phase corrected value.