A synchronous power-on/off control method in which a power-on/off command is output to a breaker (11, 12) for switching in a case where a frequency difference between a plurality of electric power supply sources (14, 18, 20A, 20B, 20C) is within a predetermined range and a phase difference between the plurality of electric power supply sources (14, 18, 20A, 20B, 20C) is within a predetermined range, in switching of electric power supply between the plurality of electric power supply sources (14, 18, 20A, 20B, 20C), at least one of the plurality of electric power supply sources (14, 18, 20A, 20B, 20C) including an electric generator which is driven by a driving power source via a transmission device (30A, 30B), the synchronous power-on/off control method comprising: detecting the frequency difference and the phase difference between the plurality of electric power supply sources (14, 18, 20A, 20B, 20C); determining whether or not the detected frequency difference is within the predetermined range, and whether or not the detected phase difference is within the predetermined range; feedback controlling a generator drive rotation speed of the transmission device (30A, 30B) so that the frequency difference is maintained at a value within the predetermined range and the phase difference is maintained at a value within the predetermined range, in a case where it is determined that the detected frequency difference is within the predetermined range and the detected phase difference is within the predetermined range; controlling the generator drive rotation speed of the transmission device by providing an offset to a rotation speed command of the transmission device (30A, 30B), in a case where it is determined that the detected frequency difference is not within the predetermined range and the detected phase difference is not within the predetermined range; and calculating a generator rotation speed command, feedback controlling the generator drive rotation speed based on the generator rotation speed command, and outputting the power-on/off command to the breaker (11, 12), in a case where it is determined that the detected frequency difference is within the predetermined range and the detected phase difference is within the predetermined range, characterized in that calculating the generator rotation speed command is provided by adding to the rotation speed command of the transmission device (30A, 30B) an output value obtained by subjecting the detected phase difference to a proportional-integral-control and a tilt motion angle of a power roller (35c, 36c) included in the transmission device (30A, 30B) is adjusted by a hydraulic servo system. detecting the frequency difference and the phase difference between the plurality of electric power supply sources (14, 18, 20A, 20B, 20C); determining whether or not the detected frequency difference is within the predetermined range, and whether or not the detected phase difference is within the predetermined range; feedback controlling a generator drive rotation speed of the transmission device (30A, 30B) so that the frequency difference is maintained at a value within the predetermined range and the phase difference is maintained at a value within the predetermined range, in a case where it is determined that the detected frequency difference is within the predetermined range and the detected phase difference is within the predetermined range; controlling the generator drive rotation speed of the transmission device by providing an offset to a rotation speed command of the transmission device (30A, 30B), in a case where it is determined that the detected frequency difference is not within the predetermined range and the detected phase difference is not within the predetermined range; and calculating a generator rotation speed command, feedback controlling the generator drive rotation speed based on the generator rotation speed command, and outputting the power-on/off command to the breaker (11, 12), in a case where it is determined that the detected frequency difference is within the predetermined range and the detected phase difference is within the predetermined range, characterized in that calculating the generator rotation speed command is provided by adding to the rotation speed command of the transmission device (30A, 30B) an output value obtained by subjecting the detected phase difference to a proportional-integral-control and a tilt motion angle of a power roller (35c, 36c) included in the transmission device (30A, 30B) is adjusted by a hydraulic servo system.