A power supply system, comprising: an electric power converter configured to convert direct-current power of a direct-current power source (1,2) to alternating-current power and then supply the alternating-current power to an electric power system; characterized in a voltage detector configured to detect a node voltage between the electric power system and the electric power converter; a current detector configured to detect an output current of the electric power converter; an effective power detector configured to obtain effective power from outputs of the voltage detector and the current detector; a system condition detector configured to detect a power supply condition in the electric power system or the operating conditions of devices included in the electric power system and then output a level detection signal based on a detection result; an effective power setting unit configured to output a preset effective power set value; a rotor speed computation unit configured to calculate an angular frequency of an output voltage of the electric power converter on the basis of an output of the effective power detector, the effective power set value, and an output of the system condition detector; an electric characteristic computation unit configured to calculate an output voltage target value of the electric power converter on the basis of the angular frequency, a current value detected by the current detector, and a set voltage value; and a constant switching module, the module being configured to switch the effective power set value to a preset first value or second value on the basis of the level detection signal, the constant switching module being configured to switch at least one of a constant used for the calculation of the angular frequency and a constant used for the calculation of the output voltage target value on the basis of the level detection signal, wherein the output voltage of the electric power converter is controlled on the basis of the output voltage target value. an electric power converter configured to convert direct-current power of a direct-current power source (1,2) to alternating-current power and then supply the alternating-current power to an electric power system; characterized in a voltage detector configured to detect a node voltage between the electric power system and the electric power converter; a current detector configured to detect an output current of the electric power converter; an effective power detector configured to obtain effective power from outputs of the voltage detector and the current detector; a system condition detector configured to detect a power supply condition in the electric power system or the operating conditions of devices included in the electric power system and then output a level detection signal based on a detection result; an effective power setting unit configured to output a preset effective power set value; a rotor speed computation unit configured to calculate an angular frequency of an output voltage of the electric power converter on the basis of an output of the effective power detector, the effective power set value, and an output of the system condition detector; an electric characteristic computation unit configured to calculate an output voltage target value of the electric power converter on the basis of the angular frequency, a current value detected by the current detector, and a set voltage value; and a constant switching module, the module being configured to switch the effective power set value to a preset first value or second value on the basis of the level detection signal, the constant switching module being configured to switch at least one of a constant used for the calculation of the angular frequency and a constant used for the calculation of the output voltage target value on the basis of the level detection signal, wherein the output voltage of the electric power converter is controlled on the basis of the output voltage target value.