A power supply system for solar power generation, comprising: - a plurality of power supply devices, wherein each power supply device comprises: - a DC/DC converter of LLC resonant type; and - an inverter connected to the DC/DC converter, wherein the DC/DC converter, when the input voltage of the DC/DC converter is smaller than a first predetermined voltage value, is configured to perform a step-up operation so that the output voltage of the DC/DC converter output to the inverter is a fixed value, and wherein the DC/DC converter, when the input voltage of the DC/DC converter is greater than a second predetermined voltage value that is set to a value greater than the first predetermined voltage value of the DC/DC converter, is configured to perform a step-down operation so that the output voltage of the DC/DC converter output to the inverter is a fixed value, wherein an output of the inverter of each power supply device is connected in series to form a Y- or a Δ-AC-grid configuration, wherein the DC/DC converter is configured such that the control frequency for driving is set to be constant in an input voltage range in which the input voltage of the DC/DC converter is larger than the first predetermined voltage value but smaller than the second predetermined voltage value such that the output voltage of the DC/DC converter changes in proportion to the input voltage. - a plurality of power supply devices, wherein each power supply device comprises: - a DC/DC converter of LLC resonant type; and - an inverter connected to the DC/DC converter, wherein the DC/DC converter, when the input voltage of the DC/DC converter is smaller than a first predetermined voltage value, is configured to perform a step-up operation so that the output voltage of the DC/DC converter output to the inverter is a fixed value, and wherein the DC/DC converter, when the input voltage of the DC/DC converter is greater than a second predetermined voltage value that is set to a value greater than the first predetermined voltage value of the DC/DC converter, is configured to perform a step-down operation so that the output voltage of the DC/DC converter output to the inverter is a fixed value, wherein an output of the inverter of each power supply device is connected in series to form a Y- or a Δ-AC-grid configuration, wherein the DC/DC converter is configured such that the control frequency for driving is set to be constant in an input voltage range in which the input voltage of the DC/DC converter is larger than the first predetermined voltage value but smaller than the second predetermined voltage value such that the output voltage of the DC/DC converter changes in proportion to the input voltage. the DC/DC converter, when the input voltage of the DC/DC converter is smaller than a first predetermined voltage value, is configured to perform a step-up operation so that the output voltage of the DC/DC converter output to the inverter is a fixed value, and wherein the DC/DC converter, when the input voltage of the DC/DC converter is greater than a second predetermined voltage value that is set to a value greater than the first predetermined voltage value of the DC/DC converter, is configured to perform a step-down operation so that the output voltage of the DC/DC converter output to the inverter is a fixed value, wherein an output of the inverter of each power supply device is connected in series to form a Y- or a Δ-AC-grid configuration, wherein the DC/DC converter is configured such that the control frequency for driving is set to be constant in an input voltage range in which the input voltage of the DC/DC converter is larger than the first predetermined voltage value but smaller than the second predetermined voltage value such that the output voltage of the DC/DC converter changes in proportion to the input voltage.