An automatic synchronous parallelization apparatus capable of suppressing an inrush current by automatically adjusting the magnitude, the frequency, and the phase of the voltage when a single-phase voltage-type DC-to-AC converting device performing an autonomous parallel operation is connected in parallel with an external single-phase AC voltage source. The automatic synchronous parallelization apparatus includes a synchronization detecting circuit, a voltage amplitude command value generating circuit, and a frequency command value generating circuit. The voltage amplitude command value generating circuit generates the 1st-axis voltage command value, and the frequency command value generating circuit generates the 2nd-axis voltage command value, based on a value relating to a frequency difference between an external single-phase AC voltage source and a single-phase voltage-type DC-to-AC converting device and an effective value of a voltage of the external single-phase AC voltage source that are detected by synchronization detecting circuit. By using such command values, an output of the single-phase voltage-type DC-to-AC converting device can be adjusted to as to coincide with a voltage waveform of the external single-phase AC voltage source.