1. A resonant inverter device comprising:a main circuit configured to convert input power supplied from a direct-current (DC) power source into alternating-current (AC) power and supply the AC power to a resonance load as output power;
an input power measurer configured to measure the input power;
a controller configured to control operations of the main circuit, the controller comprising:
an input to which a target output value that is a target value of the output power is externally input;
a deriver configured to derive a power loss or circuit efficiency of the main circuit as a conversion loss parameter of the main circuit;
an input power calculator configured to calculate an increased target output value by increasing the target output value using the conversion loss parameter, as a target value of the input power; and
an operation controller configured to control operations of the main circuit such that the calculated target value of the input power is input to the main circuit.