An impedance adjustment apparatus of the invention performs impedance matching using characteristic parameters, even where a high frequency power source of variable frequencies is used. The apparatus is applicable to a power supply system using a high frequency power source of variable frequencies. Characteristic parameters obtained by targeting a portion of combinations of position information (C) of a variable capacitor and output frequency information (F) of the power source are stored in a memory. A T-parameter acquisition unit acquires characteristic parameters corresponding to (Cnow, Fnow) at the current time. An output reflection coefficient calculation unit calculates a reflection coefficient of an output end. A target information specifying unit, based on the above information and a target input reflection coefficient, specifies target combination information in which a reflection coefficient of an output end approaches the target input reflection coefficient. Impedance matching is performed based on this information.